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
diff --git a/doc/source/triggers.rst b/doc/source/triggers.rst
index 0bd07de..17c93ab 100644
--- a/doc/source/triggers.rst
+++ b/doc/source/triggers.rst
@@ -69,6 +69,7 @@
             create = group CI Tools
             push = +force CI Tools
             pushMerge = group CI Tools
+            forgeAuthor = group CI Tools
     [access "refs/for/refs/zuul/*"]
             pushMerge = group CI Tools