Add javadoc note about bug in readParcelableCreatorInternal

follow up to b/232589966. Bug was fixed in master, but did not have time
to merge into T. Adding a note to aid developers in using the old API if
impacted.

Test: m
Bug: 240585930
Change-Id: Ibd80007b63ff8d2ad87e18c6c4d0ffedb5184ff5