KEYS: Add documentation for asymmetric keyring restrictions

Provide more specific examples of keyring restrictions as applied to
X.509 signature chain verification.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>