Document that some Telecom broadcast and extra keys are dead-ends.

Too late to @deprecate, but we can still at least indicate that the
broadcasts are dead-ends and help the developer know the right way to
go.

Test: make -j offline-sdk-docs; confirm docs look good.
Bug: 148268693
Fixes: 152915447
Change-Id: I88b03a62c812232cf0d5bbeb693cdaf3c2eed3e1