add forge author identity privilege to support pushing refs to gerrit

without forge author identity privilege, zuul will fail to push
refs/zuul/master/**** to gerrit because zuul git user is different
from the commit's author. Therefore, update the documentation.

Fixes bug: 1226877

Change-Id: I3f59f03e28578e61a748e6f33a25a5707f433a1b
Merge "add forge author identity privilege to support pushing refs to gerrit"