Split documentation on string syntax out of "character constants"

Split the documentation on string syntax, as a lexical entity, out of
the section about character constants.  It was confusing to have
discussion that apply more to string constants than character
constants, and they use the same syntax anyway.

doc/nasmdoc.src

@@ -1396,9 +1396,9 @@ Some examples:
 \c         mov     ax,1001_0011b   ; same binary constant
 
 
-\S{chrconst} \i{Character Constants}
+\S{strings} \I{Strings}\i{Character Strings}
 
-A character constant consists of up to four characters enclosed in
+A character string consists of up to eight characters enclosed in
 either single quotes (\c{'...'}), double quotes (\c{"..."}) or
 backquotes (\c{`...`}).  Single or double quotes are equivalent to
 NASM (except of course that surrounding the constant with single
@@ -1406,17 +1406,6 @@ quotes allows double quotes to appear within it and vice versa); the
 contents of those are represented verbatim.  Strings enclosed in
 backquotes support C-style \c{\\}-escapes for special characters.
 
-A character constant with more than one character will be arranged
-with \i{little-endian} order in mind: if you code
-
-\c           mov eax,'abcd'
-
-then the constant generated is not \c{0x61626364}, but
-\c{0x64636261}, so that if you were then to store the value into
-memory, it would read \c{abcd} rather than \c{dcba}. This is also
-the sense of character constants understood by the Pentium's
-\i\c{CPUID} instruction.
-\# (see \k{insCPUID})
 
 The following escape sequences are recognized by backquoted strings:
 
@@ -1449,11 +1438,30 @@ UTF-8.  For example, the following lines are all equivalent:
 \c       db 0E2h, 098h, 0BAh    ; UTF-8 smiley face
 
 
-\S{strconst} String Constants
+\S{chrconst} \i{Character Constants}
 
-String constants are only acceptable to some pseudo-instructions,
-namely the \I\c{DW}\I\c{DD}\I\c{DQ}\I\c{DT}\I\c{DO}\I\c{DY}\i\c{DB}
-family and \i\c{INCBIN}.
+A character constant consists of a string up to eight bytes long, used
+in an expression context.  It is treated as if it was an integer.
+
+A character constant with more than one byte will be arranged
+with \i{little-endian} order in mind: if you code
+
+\c           mov eax,'abcd'
+
+then the constant generated is not \c{0x61626364}, but
+\c{0x64636261}, so that if you were then to store the value into
+memory, it would read \c{abcd} rather than \c{dcba}. This is also
+the sense of character constants understood by the Pentium's
+\i\c{CPUID} instruction.
+
+
+\S{strconst} \i{Character Constants}
+
+String constants are character strings used in the context of some
+pseudo-instructions, namely the
+\I\c{DW}\I\c{DD}\I\c{DQ}\I\c{DT}\I\c{DO}\I\c{DY}\i\c{DB} family and
+\i\c{INCBIN} (where it represents a filename.)  They are also used by
+certain preprocessor directives.
 
 A string constant looks like a character constant, only longer. It
 is treated as a concatenation of maximum-size character constants
@@ -1468,12 +1476,12 @@ And the following are also equivalent:
 \c       dd    'nine','char','s'     ; becomes three doublewords
 \c       db    'ninechars',0,0,0     ; and really looks like this
 
-Note that when used as operands to the \c{DB} family
-pseudo-instructions, quoted strings are treated as a string constants
-even if they are short enough to be a character constant, because
-otherwise \c{db 'ab'} would have the same effect as \c{db 'a'}, which
-would be silly. Similarly, three-character or four-character constants
-are treated as strings when they are operands to \c{DW}, and so forth.
+Note that when used in a string-supporting context, quoted strings are
+treated as a string constants even if they are short enough to be a
+character constant, because otherwise \c{db 'ab'} would have the same
+effect as \c{db 'a'}, which would be silly. Similarly, three-character
+or four-character constants are treated as strings when they are
+operands to \c{DW}, and so forth.
 
 
 \S{fltconst} \I{floating-point, constants}Floating-Point Constants