futex: add requeue-pi documentation

Add Documentation/futex-requeue-pi.txt describing the motivation for the
newly added FUTEX_*REQUEUE_PI op codes and their implementation.

[ Impact: add documentation ]

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Sripathi Kodi <sripathik@in.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Cc: Jakub Jelinek <jakub@redhat.com>
LKML-Reference: <4A03634E.3080609@us.ibm.com>
[ reformatted the file ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>