Improve documentation of the 'function' special form

Point out that 'function' quoting is beneficial also for symbols.

* src/eval.c (function): Enhance docstring.
* doc/lispref/functions.texi (Anonymous Functions): Improve
documentation.

doc/lispref/functions.texi

@@ -1122,6 +1122,10 @@ a byte-code function object (@pxref{Byte Compilation}).
 When lexical binding is enabled, @var{function-object} is converted
 into a closure.  @xref{Closures}.
 @end itemize
+
+When @var{function-object} is a symbol and the code is byte compiled,
+the byte-compiler will warn if that function is not defined or might
+not be known at run time.
 @end defspec
 
 @cindex @samp{#'} syntax

src/eval.c

@@ -544,8 +544,8 @@ usage: (quote ARG)  */)
 
 DEFUN ("function", Ffunction, Sfunction, 1, UNEVALLED, 0,
        doc: /* Like `quote', but preferred for objects which are functions.
-In byte compilation, `function' causes its argument to be compiled.
-`quote' cannot do that.
+In byte compilation, `function' causes its argument to be handled by
+the byte compiler.  `quote' cannot do that.
 usage: (function ARG)  */)
   (Lisp_Object args)
 {