Add documentation for SECRET_CODE_ACTION according to API review

process.

 * Document who can send and who can receive the broadcast. What are the security restrictions?
   The implication here is that there should be some. What are the wake-up semantics?
 * Document who can receive SECRET_CODE action and under what circumstances,
   and what permissions are needed to receive and send it.

Bug: 73751267, 73392896
Test: Basic telephony sanity
Change-Id: I1b6138c9ddf4cb3d84d8b652e18d4e57f410ee4d