Improve documentation of functions that accept time values

* doc/lispref/os.texi (Time Calculations): Mention the meaning of
'nil' or a scalar number as the time-value argument.  Add a
cross-reference to 'float-time' for computing a time difference as
a scalar number of seconds.

* src/editfns.c (Fformat_time_string, Ftime_less_p)
(Ftime_subtract, Ftime_add, Fdecode_time, Fcurrent_time_string)
(Fcurrent_time_zone): Mention in the doc strings the meaning of
nil argument and the fact that a time value can be a scalar number
of seconds since the epoch.
(Ftime_subtract): Mention 'float-time'.

doc/lispref/os.texi

@@ -1472,7 +1472,7 @@ corresponding time value.
 @defun format-time-string format-string &optional time zone
 
 This function converts @var{time} (or the current time, if
-@var{time} is omitted) to a string according to
+@var{time} is omitted or @code{nil}) to a string according to
 @var{format-string}.  The conversion uses the time zone rule @var{zone}, which
 defaults to the current time zone rule.  @xref{Time Zone Rules}.  The argument
 @var{format-string} may contain @samp{%}-sequences which say to
@@ -1690,7 +1690,9 @@ interactively, it prints the duration in the echo area.
 @cindex calendrical computations
 
   These functions perform calendrical computations using time values
-(@pxref{Time of Day}).
+(@pxref{Time of Day}).  A value of @code{nil} for any of their
+time-value arguments stands for the current system time, and a single
+integer number stands for the number of seconds since the epoch.
 
 @defun time-less-p t1 t2
 This returns @code{t} if time value @var{t1} is less than time value
@@ -1699,12 +1701,15 @@ This returns @code{t} if time value @var{t1} is less than time value
 
 @defun time-subtract t1 t2
 This returns the time difference @var{t1} @minus{} @var{t2} between
-two time values, as a time value.
+two time values, as a time value.  If you need the difference in units
+of elapsed seconds, use @code{float-time} (@pxref{Time of Day,
+float-time}) to convert the result into seconds.
 @end defun
 
 @defun time-add t1 t2
 This returns the sum of two time values, as a time value.
-One argument should represent a time difference rather than a point in time.
+One argument should represent a time difference rather than a point in time,
+either as a list or as a single number of elapsed seconds.
 Here is how to add a number of seconds to a time value:
 
 @example

src/editfns.c

@@ -1581,21 +1581,28 @@ time_arith (Lisp_Object a, Lisp_Object b,
 }
 
 DEFUN ("time-add", Ftime_add, Stime_add, 2, 2, 0,
-       doc: /* Return the sum of two time values A and B, as a time value.  */)
+       doc: /* Return the sum of two time values A and B, as a time value.
+A nil value for either argument stands for the current time.
+See `current-time-string' for the various forms of a time value.  */)
   (Lisp_Object a, Lisp_Object b)
 {
   return time_arith (a, b, time_add);
 }
 
 DEFUN ("time-subtract", Ftime_subtract, Stime_subtract, 2, 2, 0,
-       doc: /* Return the difference between two time values A and B, as a time value.  */)
+       doc: /* Return the difference between two time values A and B, as a time value.
+Use `float-time' to convert the difference into elapsed seconds.
+A nil value for either argument stands for the current time.
+See `current-time-string' for the various forms of a time value.  */)
   (Lisp_Object a, Lisp_Object b)
 {
   return time_arith (a, b, time_subtract);
 }
 
 DEFUN ("time-less-p", Ftime_less_p, Stime_less_p, 2, 2, 0,
-       doc: /* Return non-nil if time value T1 is earlier than time value T2.  */)
+       doc: /* Return non-nil if time value T1 is earlier than time value T2.
+A nil value for either argument stands for the current time.
+See `current-time-string' for the various forms of a time value.  */)
   (Lisp_Object t1, Lisp_Object t2)
 {
   int t1len, t2len;
@@ -1973,11 +1980,13 @@ emacs_nmemftime (char *s, size_t maxsize, const char *format,
 }
 
 DEFUN ("format-time-string", Fformat_time_string, Sformat_time_string, 1, 3, 0,
-       doc: /* Use FORMAT-STRING to format the time TIME, or now if omitted.
+       doc: /* Use FORMAT-STRING to format the time TIME, or now if omitted or nil.
 TIME is specified as (HIGH LOW USEC PSEC), as returned by
-`current-time' or `file-attributes'.  The obsolete form (HIGH . LOW)
+`current-time' or `file-attributes'.
-is also still accepted.  The optional ZONE is omitted or nil for Emacs
+It can also be a single integer number of seconds since the epoch.
-local time, t for Universal Time, `wall' for system wall clock time,
+The obsolete form (HIGH . LOW) is also still accepted.
+The optional ZONE is omitted or nil for Emacs local time,
+t for Universal Time, `wall' for system wall clock time,
 or a string as in the TZ environment variable.
 
 The value is a copy of FORMAT-STRING, but with certain constructs replaced
@@ -2094,7 +2103,9 @@ DEFUN ("decode-time", Fdecode_time, Sdecode_time, 0, 2, 0,
        doc: /* Decode a time value as (SEC MINUTE HOUR DAY MONTH YEAR DOW DST UTCOFF).
 The optional SPECIFIED-TIME should be a list of (HIGH LOW . IGNORED),
 as from `current-time' and `file-attributes', or nil to use the
-current time.  The obsolete form (HIGH . LOW) is also still accepted.
+current time.
+It can also be a single integer number of seconds since the epoch.
+The obsolete form (HIGH . LOW) is also still accepted.
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
 the TZ environment variable.
@@ -2219,8 +2230,10 @@ which provide a much more powerful and general facility.
 If SPECIFIED-TIME is given, it is a time to format instead of the
 current time.  The argument should have the form (HIGH LOW . IGNORED).
 Thus, you can use times obtained from `current-time' and from
-`file-attributes'.  SPECIFIED-TIME can also have the form (HIGH . LOW),
+`file-attributes'.  SPECIFIED-TIME can also be a single integer
-but this is considered obsolete.
+number of seconds since the epoch.
+SPECIFIED-TIME can also have the form (HIGH . LOW), but this is
+considered obsolete.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
 Universal Time, `wall' for system wall clock time, or a string as in
@@ -2298,8 +2311,9 @@ NAME is a string giving the name of the time zone.
 If SPECIFIED-TIME is given, the time zone offset is determined from it
 instead of using the current time.  The argument should have the form
 \(HIGH LOW . IGNORED).  Thus, you can use times obtained from
-`current-time' and from `file-attributes'.  SPECIFIED-TIME can also
+`current-time' and from `file-attributes'.  SPECIFIED-TIME can also be
-have the form (HIGH . LOW), but this is considered obsolete.
+a single integer number of seconds since the epoch.  SPECIFIED-TIME can
+also have the form (HIGH . LOW), but this is considered obsolete.
 Optional second arg ZONE is omitted or nil for the local time zone, or
 a string as in the TZ environment variable.