Update system group documentation
Update system group documentation

Administrators and batch users are not system groups any more, and
the identity of system groups cannot be set in `system_config`.

Change-Id: I1ec4d3e316115a669362d670f6e1eb7107aa2848

anminhsu/gerrit,GerritCodeReview/gerrit,Saulis/gerrit,joshuawilson/merrit,Overruler/gerrit,supriyantomaftuh/gerrit,hdost/gerrit,renchaorevee/gerrit,Team-OctOS/host_gerrit,gcoders/gerrit,joshuawilson/merrit,quyixia/gerrit,netroby/gerrit,renchaorevee/gerrit,GerritCodeReview/gerrit,renchaorevee/gerrit,GerritCodeReview/gerrit,joshuawilson/merrit,pkdevbox/gerrit,jackminicloud/test,Seinlin/gerrit,MerritCR/merrit,gcoders/gerrit,supriyantomaftuh/gerrit,jackminicloud/test,TonyChai24/test,Distrotech/gerrit,quyixia/gerrit,thinkernel/gerrit,jackminicloud/test,gcoders/gerrit,dwhipstock/gerrit,supriyantomaftuh/gerrit,Seinlin/gerrit,qtproject/qtqa-gerrit,hdost/gerrit,gerrit-review/gerrit,supriyantomaftuh/gerrit,quyixia/gerrit,dwhipstock/gerrit,Seinlin/gerrit,Seinlin/gerrit,anminhsu/gerrit,WANdisco/gerrit,pkdevbox/gerrit,Saulis/gerrit,qtproject/qtqa-gerrit,renchaorevee/gerrit,Overruler/gerrit,Seinlin/gerrit,quyixia/gerrit,WANdisco/gerrit,pkdevbox/gerrit,MerritCR/merrit,pkdevbox/gerrit,joshuawilson/merrit,qtproject/qtqa-gerrit,joshuawilson/merrit,thesamet/gerrit,gcoders/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,quyixia/gerrit,anminhsu/gerrit,dwhipstock/gerrit,jackminicloud/test,pkdevbox/gerrit,WANdisco/gerrit,dwhipstock/gerrit,gcoders/gerrit,hdost/gerrit,Overruler/gerrit,thinkernel/gerrit,Saulis/gerrit,MerritCR/merrit,Distrotech/gerrit,gcoders/gerrit,TonyChai24/test,anminhsu/gerrit,joshuawilson/merrit,renchaorevee/gerrit,gerrit-review/gerrit,quyixia/gerrit,MerritCR/merrit,netroby/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,supriyantomaftuh/gerrit,Team-OctOS/host_gerrit,WANdisco/gerrit,Saulis/gerrit,pkdevbox/gerrit,supriyantomaftuh/gerrit,joshuawilson/merrit,thesamet/gerrit,TonyChai24/test,hdost/gerrit,thinkernel/gerrit,MerritCR/merrit,Seinlin/gerrit,Overruler/gerrit,Overruler/gerrit,netroby/gerrit,pkdevbox/gerrit,dwhipstock/gerrit,qtproject/qtqa-gerrit,netroby/gerrit,GerritCodeReview/gerrit,MerritCR/merrit,TonyChai24/test,Distrotech/gerrit,TonyChai24/test,Team-OctOS/host_gerrit,joshuawilson/merrit,Distrotech/gerrit,Team-OctOS/host_gerrit,anminhsu/gerrit,hdost/gerrit,netroby/gerrit,TonyChai24/test,thinkernel/gerrit,qtproject/qtqa-gerrit,Team-OctOS/host_gerrit,netroby/gerrit,thinkernel/gerrit,TonyChai24/test,gerrit-review/gerrit,renchaorevee/gerrit,WANdisco/gerrit,thesamet/gerrit,anminhsu/gerrit,quyixia/gerrit,hdost/gerrit,anminhsu/gerrit,Team-OctOS/host_gerrit,supriyantomaftuh/gerrit,GerritCodeReview/gerrit,Saulis/gerrit,MerritCR/merrit,Distrotech/gerrit,thesamet/gerrit,qtproject/qtqa-gerrit,jackminicloud/test,Seinlin/gerrit,Team-OctOS/host_gerrit,thinkernel/gerrit,hdost/gerrit,MerritCR/merrit,thinkernel/gerrit,qtproject/qtqa-gerrit,Overruler/gerrit,renchaorevee/gerrit,jackminicloud/test,Saulis/gerrit,WANdisco/gerrit,dwhipstock/gerrit,netroby/gerrit,thesamet/gerrit,gerrit-review/gerrit,thesamet/gerrit,thesamet/gerrit,Distrotech/gerrit,gerrit-review/gerrit,dwhipstock/gerrit,Distrotech/gerrit,gcoders/gerrit,gerrit-review/gerrit,jackminicloud/test