Merge branch 'rw-lock-docs' into 'master'

gthread: Clarify priority handling in GRWLock

See merge request GNOME/glib!376

glib/gthread.c

@@ -320,6 +320,10 @@
  * simultaneous read-only access (by holding the 'reader' lock via
  * g_rw_lock_reader_lock()).
  *
+ * It is unspecified whether readers or writers have priority in acquiring the
+ * lock when a reader already holds the lock and a writer is queued to acquire
+ * it.
+ *
  * Here is an example for an array with access functions:
  * |[<!-- language="C" --> 
  *   GRWLock lock;