[glib] gmain: Document threading properties of g_source_is_destroyed()

[Philip Withnall <pwithnall src gnome org>]

commit c408256b5236c1cf6e85c25be95ed5a73b5c789b
Author: Philip Withnall <withnall endlessm com>
Date:   Sun Feb 5 16:40:33 2017 +0100

    gmain: Document threading properties of g_source_is_destroyed()
    
    https://bugzilla.gnome.org/show_bug.cgi?id=778049

 glib/gmain.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index 5aea34f..7d23d7c 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -3063,6 +3063,12 @@ g_main_current_source (void)
  * }
  * ]|
  *
+ * Calls to this function from a thread other than the one acquired by the
+ * #GMainContext the #GSource is attached to are typically redundant, as the
+ * source could be destroyed immediately after this function returns. However,
+ * once a source is destroyed it cannot be un-destroyed, so this function can be
+ * used for opportunistic checks from any thread.
+ *
  * Returns: %TRUE if the source has been destroyed
  *
  * Since: 2.12