docs: Info on two special (more-than-'dangerous') permissions

There are a couple of 'signature' permissions that have a special
user-authorization process; mentioned them and put in a link to their
reference docs.

The reference docs for those methods is being updated in a separate
CL, http://ag/784299

See first comment for doc stage location.

bug: 24673125
Change-Id: I4f17784b570d6154482e3fe965bd2de1043babb6