add comment about using not quite standard mbstowcs() feature

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@36737 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/wxchar.cpp

@@ -78,6 +78,12 @@ size_t WXDLLEXPORT wxMB2WC(wchar_t *buf, const char *psz, size_t n)
 #endif
   }
 
+  // note that we rely on common (and required by Unix98 but unfortunately not
+  // C99) extension which allows to call mbs(r)towcs() with NULL output pointer
+  // to just get the size of the needed buffer -- this is needed as otherwise
+  // we have no idea about how much space we need and if the CRT doesn't
+  // support it (the only currently known example being Metrowerks, see
+  // wx/wxchar.h) we don't use its mbstowcs() at all
 #ifdef HAVE_WCSRTOMBS
   return mbsrtowcs((wchar_t *) NULL, &psz, 0, &mbstate);
 #else