Add Change Owner group to documentation

Gerrit would not allow assignment of the Change Owner group using git push
because this group isn't written to the groups file by default.  Adding the
group to the documentation will hopefully help users figure out how to
make it work.

Bug: issue 2388
Change-Id: Ib57b306a749bdf2817cd9c2e47a824636945b6bc
Merge "Add Change Owner group to documentation"