Update Identity Credential API docs.

This change contains no actual syntactical or semantic changes, just
clarifications on the inputs and outputs.

Test: N/A
Bug: 151082886
Merged-In: Ic7797aa53d292abdeb779cb55b404f8a433bce79
Change-Id: I90279c1ba434a9305b991863086b867309549ce8
4 files changed