Add some warnings and notes about framework's jarjar rules

These rules are explicitly recommended  to be depended on by other
libraries in the bootclasspath, that may depend on the framework's
copy of the hidl library.

Bug: 186804505
Test: presubmit
Merged-In: I09766423782d9bb9408f3d14169d7e5d8f1b5823
Change-Id: I09766423782d9bb9408f3d14169d7e5d8f1b5823
(cherry picked from commit 524bf249)