[glib] Clarify g_propagate_error docs

[Matthias Clasen <matthiasc src gnome org>]

commit 26c66ab1b9806d097bd1080240435cb37642f141
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Oct 10 14:17:56 2014 -0400

    Clarify g_propagate_error docs
    
    I just ran into a bug that was caused by having src being a
    persistent GError* that was not cleared after propagating it.

 glib/gerror.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/glib/gerror.c b/glib/gerror.c
index db31692..4eed58e 100644
--- a/glib/gerror.c
+++ b/glib/gerror.c
@@ -604,6 +604,10 @@ g_set_error_literal (GError      **err,
  *
  * If @dest is %NULL, free @src; otherwise, moves @src into * dest 
  * The error variable @dest points to must be %NULL.
+ *
+ * Note that @src is no longer valid after this call. If you want
+ * to keep using the same GError*, you need to set it to %NULL
+ * after calling this function on it.
  */
 void
 g_propagate_error (GError **dest,