Update groups rest api doc to explain the ?m= param
Update groups rest api doc to explain the ?m= param

Adds doc for ?m= param.

Change-Id: Ie2679ace999eb0dbdc4db280169aaed97fbbac52
(cherry picked from commit 3cbb53005cf5369a78628975662070cd2791bd10)

qtproject/qtqa-gerrit,gerrit-review/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,gerrit-review/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit