Update Docstrings for #requestLocationUpdate()

Address API Council feedback for CellLocation#requestLocationUpdate

-Update the summary string to indicate the functionality
 for pre-SDK31 behavior.
-Clarify that this API is only useful in power-saving modes.
-Remove justification for deprecation leaving a simple link to
 a superior alternative.

Bug: 161207204
Test: make offline-sdk-docs && make (docstring only change)
Change-Id: Ib931906323fe1ec2c730271cefd0d46aee60d963