Add documentation of Build.MODEL addition to mms uaprof and ua

To support simpler (less) configuration of mms user agent and mms
user agent profile url we have added support to read Build.MODEL
for PhoneInterfaceManager. In some cases this would mean one
configuration per oem.

Flag: EXEMPT bug fix
Bug: 298273889
Test: atest PhoneInterfaceManagerTest
Change-Id: If8a0049205a8882b0cddbef1e9388a9d5e52de97