Update CellInfo Documentation on Updates

-Clarify that updates to CellInfo are not guaranteed
 regardless of SDK level.
-Suggest that callers use getTimeStamp() to see how
 old/new the cached CellInfo data is.

Bug: 122617476
Test: make offline-sdk-docs (docstring only change)
Change-Id: Ie9f3597ab2ba027179ce3c37cfeb551e8f659886