Clarify capabilities granted by CALL_PHONE permission.

Make it clear in the permission description string that CALL_PHONE can
put used to change call forwarding settings.

Also clarify API docs to indicate that MMI codes are dialable.

Bug: 268341970
Test: Docs only change.
Change-Id: Ia33c267ae262d5ed4c7eaad7f2f3c942b44252ad