Docstring Updates for applyTransportModeTransform

-Add anotations to usages of PolicyDirection for
 apply...() methods.
-Update the comments on DIRECTION_IN and DIRECTION_OUT
 to better reflect their current usage.
-Add a better explanation to the rekey procedure doc.
-Remove disused createTunnelInterface() stub.

Bug: 73751066
Test: make docs
Change-Id: I9f2ec864466148a18899f1e952c74a525902ccbc