Correct to docs for g_strcmp0

Don't promise to return -1 or 1 - we fall back to strcmp which
does not guarantee this.
https://bugzilla.gnome.org/show_bug.cgi?id=685037

glib/gtestutils.c

@@ -1977,7 +1977,7 @@ g_assertion_message_error (const char     *domain,
  * gracefully by sorting it before non-%NULL strings.
  * Comparing two %NULL pointers returns 0.
  *
- * Returns: -1, 0 or 1, if @str1 is <, == or > than @str2.
+ * Returns: an integer less than, equal to, or greater than zero, if @str1 is <, == or > than @str2.
  *
  * Since: 2.16
  */