pkeys: Add details of system call use to Documentation/

This spells out all of the pkey-related system calls that we have
and provides some example code fragments to demonstrate how we
expect them to be used.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: Dave Hansen <dave@sr71.net>
Cc: mgorman@techsingularity.net
Cc: arnd@arndb.de
Cc: linux-api@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: luto@kernel.org
Cc: akpm@linux-foundation.org
Cc: torvalds@linux-foundation.org
Link: http://lkml.kernel.org/r/20160729163020.59350E33@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>