Modify docs to clarify rules around placing calls to CF MMI codes

Clarify in the docs that only the system/default dialer can
directly place call forwarding MMI codes. For other applications,
placing a call using these mmi codes will cause the dialer to
pop up with the MMI code populated so that the user can manually
confirm  before the call is placed.

Bug: 306380262
Test: docs only change
Flag: DOCS_ONLY
Change-Id: Id486304020b12e01785c5d98a1496d3bd9cfac43