Update Docstring for SmsMessage.getOriginatingAddress

There was a public request for clarification on the address
format for getOriginatingAddress. I did a little research and
have added the answer to the docstring.

Bug: 64697463
Test: compilation
Merged-In: Icf37af0a5940a6fb7798d7c7cafe7b97683bb689
Change-Id: Icf37af0a5940a6fb7798d7c7cafe7b97683bb689