Documentation/protection-keys: Clean up documentation for User Space pkeys

The documentation for user space pkeys was a bit dated including things
such as Amazon and distribution testing information which is irrelevant
now.

Update the documentation.  This also streamlines adding the Supervisor
pkey documentation later on.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220419170649.1022246-2-ira.weiny@intel.com