keys: update the documentation with info about "logon" keys

The key service provides a number of features besides keys:

 (*) The key service defines two special key types:

 (*) The key service defines three special key types:

     (+) "keyring"

	 blobs of data. These can be created, updated and read by userspace,

	 and aren't intended for use by kernel services.

     (+) "logon"

	 Like a "user" key, a "logon" key has a payload that is an arbitrary

	 blob of data. It is intended as a place to store secrets which are

	 accessible to the kernel but not to userspace programs.

	 The description can be arbitrary, but must be prefixed with a non-zero

	 length string that describes the key "subclass". The subclass is

	 separated from the rest of the description by a ':'. "logon" keys can

	 be created and updated from userspace, but the payload is only

	 readable from kernel space.

 (*) Each process subscribes to three keyrings: a thread-specific keyring, a

     process-specific keyring, and a session-specific keyring.