Fix docstrings for RIL_CellIdentity

Update the documentation to match the current
CTS requirements for Cell Identity reporting
-For LTE, WCDMA, and GSM require that the channel
 number be reported.
-For LTE and WCDMA, require that the correct
 physical channel identifier be reported. Note that
 due to b/32774471 we still cannot require BSIC
 to be reported.

Bug: 65174915
Test: none, comment-only change
Change-Id: I93fcce3b39401e8965f724722361c2cadc4a88cc