Revert deprecation of Telephony FDE constant

The API council doesn't want this constant be deprecated.  This is a
partial revert of http://ag/17812636, which keeps and revises the text
that was added to the documentation.

Bug: 208476087
Bug: 232310299
Change-Id: I16b8c15288963d7c3318e8c614f0f7f0b091c221