Doc: Add info about Gerrit UI extension point
Doc: Add info about Gerrit UI extension point

Change I10fe9acf25ee6 added the CHANGE_SCREEN_BELOW_RELATED_INFO_BLOCK
extension point but forgot to provide documentation for it.

Change-Id: Ib5bcfac330d110b950325775ae69ef6b8810ae0e

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