Improve documentation for unlockUserKeyIfUnsecured()

Document more clearly what unlockUserKeyIfUnsecured() actually does.  It
probably should be renamed to something clearer (I'm thinking of just
changing "Key" to "Keys"), but this is a start.

Bug: 306204742
Flag: exempt, comment-only change
Test: presubmit
Change-Id: Ib35a62fb4627db5755fac57609b1fbb3a659492d