static keys: Add docs better explaining the whole 'struct static_key' mechanism

Add better documentation for static keys.

Signed-off-by: Jason Baron <jbaron@redhat.com>
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: davem@davemloft.net
Cc: ddaney.cavm@gmail.com
Cc: a.p.zijlstra@chello.nl
Link: http://lkml.kernel.org/r/52570e566e5f1914f27b67e4eafb5781b8f9f9db.1329851692.git.jbaron@redhat.com
[ Added a 'Summary' section and rewrote it to explain static keys ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>