Add InCallService docs to state prerequisites for be being a dialer app.

The InCallService documentation does not clearly state that an app which
wants to become the default dialer app also needs to handle the DIAL
intent.  This change updates the documentation to make this clear.

Test: Built documentation and verified its contents.
Bug: 72857373
Change-Id: I738a8475df37035a0a7f4ad1639df76d967f81dc