locking/lglocks: Add documentation of current lglocks implementation

Local/global locks are currently not documented anywhere other
than in an somewhat out-of-date LWN article - this is an attempt
to document the current state of lglocks.

This patch is against linux-next 3.18.0-rc6

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Carsten Emde <c.emde@osadl.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20141208083326.GA29895@opentech.at
Signed-off-by: Ingo Molnar <mingo@kernel.org>