https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41041
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
SOmething like this:
--- a/gcc/doc/cppopts.texi
+++ b/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}.
