Update docs to hint when to use FLAG_MUTABLE

Any PI that needs to be used with inline reply or bubbles needs to keep
their PI mutable. This change updates the messaging around FLAG_MUTABLE
docs and logs at the mutability flag check to add this use
case.

Bug: 160794467
Test: docs change, green builds
Change-Id: I641b35f92de5843be2fad3ef4621ffc9c6456899
1 file changed