Improve g_cond_wait docs

Document that the mutex is locked upon return.
https://bugzilla.gnome.org/show_bug.cgi?id=691110

glib/gthread-posix.c

@@ -727,6 +727,8 @@ g_cond_clear (GCond *cond)
  * @mutex: a #GMutex that is currently locked
  *
  * Atomically releases @mutex and waits until @cond is signalled.
+ * When this function returns, @mutex is locked again and owned by the
+ * calling thread.
  *
  * When using condition variables, it is possible that a spurious wakeup
  * may occur (ie: g_cond_wait() returns even though g_cond_signal() was