doc: Document correct -fwide-exec-charset defaults [PR41041]

As shown in the PR, the default is not UTF-32 but rather UTF-32BE or
UTF-32LE, avoiding the need for a byte order mark in literals.

gcc/ChangeLog:

	PR c/41041
	* doc/cppopts.texi: Document -fwide-exec-charset defaults
	correctly.

gcc/doc/cppopts.texi

@@ -318,9 +318,10 @@ supported by the system's @code{iconv} library routine.
 @opindex fwide-exec-charset
 @cindex character set, wide execution
 Set the wide execution character set, used for wide string and
-character constants.  The default is UTF-32 or UTF-16, whichever
-corresponds to the width of @code{wchar_t}.  As with
-@option{-fexec-charset}, @var{charset} can be any encoding supported
+character constants.  The default is one of UTF-32BE, UTF-32LE, UTF-16BE,
+or UTF-16LE, whichever corresponds to the width of @code{wchar_t} and the
+big-endian or little-endian byte order being used for code generation.  As
+with @option{-fexec-charset}, @var{charset} can be any encoding supported
 by the system's @code{iconv} library routine; however, you will have
 problems with encodings that do not fit exactly in @code{wchar_t}.