Doc: glib: Fix all undocumented/unused/undeclared symbols

There is one issue left in gscanner.h due to a bug #741305 in gtk-doc.

https://bugzilla.gnome.org/show_bug.cgi?id=740814

glib/gstdio.c

@@ -845,6 +845,8 @@ g_utime (const gchar    *filename,
  * attempt to correctly handle %EINTR, which has platform-specific
  * semantics.
  *
+ * Returns: %TRUE on success, %FALSE if there was an error.
+ *
  * Since: 2.36
  */
 gboolean