gthread: Fix g_rw_lock_reader_lock() documentation
gthread: Fix g_rw_lock_reader_lock() documentation

The documentation erroneously said that a thread calling
g_rw_lock_reader_lock() would always block if another thread was waiting
for the write lock. That’s not true: if no thread holds the lock, it is
implementation defined which of a waiting reader and writer gets the
lock.

See
http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_rwlock_rdlock.html.

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

Fixes: #590

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib