dm crypt: document encrypted keyring key option

Commit 27f5411a71 ("dm crypt: support using encrypted keys")
introduced support for encrypted keyring type.

Fix documentation in admin guide to mention this type.

Fixes: 27f5411a71 ("dm crypt: support using encrypted keys")
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Documentation/admin-guide/device-mapper/dm-crypt.rst

@@ -67,7 +67,7 @@ Parameters::
     the value passed in <key_size>.
 
 <key_type>
-    Either 'logon' or 'user' kernel key type.
+    Either 'logon', 'user' or 'encrypted' kernel key type.
 
 <key_description>
     The kernel keyring key description crypt target should look for