docs: Document not to use volatile qualifiers

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #600

glib/gthread.c

@@ -686,6 +686,9 @@ g_once_impl (GOnce       *once,
  *   // use initialization_value here
  * ]|
  *
+ * While @location has a `volatile` qualifier, this is a historical artifact and
+ * the pointer passed to it should not be `volatile`.
+ *
  * Returns: %TRUE if the initialization section should be entered,
  *     %FALSE and blocks otherwise
  *
@@ -725,6 +728,9 @@ gboolean
  * releases concurrent threads blocking in g_once_init_enter() on this
  * initialization variable.
  *
+ * While @location has a `volatile` qualifier, this is a historical artifact and
+ * the pointer passed to it should not be `volatile`.
+ *
  * Since: 2.14
  */
 void