Add missing documentation of secondary index configuration
Add missing documentation of secondary index configuration

Document that open and closed changes are indexed in separate
indexes, and for Lucene indexes the RAM buffer size and maximum
buffered documents can be configured.

Add an example configuration.

Change-Id: Ic4cb834cea0c716052c419377a818e369bfe2ce7

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