gthread: Clarify priority handling in GRWLock

As we use pthread_rwlock_*() to implement GRWLock (on Unix), the
priority of readers vs writers when trying to acquire a lock already
held by one reader with a writer queued, is unspecified. i.e. We don’t
explicitly prioritise the pending readers to acquire the lock (and block
the writer), or vice-versa.

Whatever our implementation on other platforms, we must document the
priority as unspecified, as that’s what happens on Unix and is the
least restrictive API guarantee we can make.

Prompted by https://stackoverflow.com/q/52661672/2931197.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

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;