Update the docs for FLAG_UPDATE_CURRENT and FLAG_IMMUTABLE

to add how they interact with each other. This seems to be a common
confusion for users. This change adds more context around what the two
flags mean in terms of allowing changes to a PendingIntent and that it
should be fine to use both together.

Bug: 160794467
Test: Green builds
Change-Id: I66f3744ca95252bdfebf8c9711eec7e8bfd059c8