[glib: 1/2] gmain: Clarify documentation for g_source_get_id()

[Philip Withnall <pwithnall src gnome org>]

commit 515dac258cf93315b5bd4ab29eb086e144378dd0
Author: Philip Withnall <withnall endlessm com>
Date:   Thu Jun 21 17:09:57 2018 +0100

    gmain: Clarify documentation for g_source_get_id()
    
    It’s good to know *which* GMainContext is used to determine the ID, and
    the preconditions for calling this method.
    
    Using wording suggested by Emmanuele Bassi <ebassi gmail com>.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>

 glib/gmain.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index 7d49025c7..5f0b05198 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -1296,6 +1296,11 @@ g_source_destroy (GSource *source)
  * context. The reverse
  * mapping from ID to source is done by g_main_context_find_source_by_id().
  *
+ * You can only call this function while the source is associated to a
+ * #GMainContext instance; calling this function before g_source_attach()
+ * or after g_source_destroy() yields undefined behavior. The ID returned
+ * is unique within the #GMainContext instance passed to g_source_attach().
+ *
  * Returns: the ID (greater than 0) for the source
  **/
 guint