Document key-value tree transforms and improve checks

Document key-value tree transforms and improve checks

Add assertions to provide better messages in case of unintended usage,
and document the interface used to specify rules.  This should cover all
of the key-value tree code that is currently exposed for mdp input
handling through IInputRecExtension, with the exception of
KeyValueTreeObjectBuilder that is exposed if one uses toObject().

Change-Id: Ic36a2c41ba418abfa46cc9b37d6cd7eed5a896b6