Replace documentation of gerrit.ui with gerrit.enableGwtUi
Replace documentation of gerrit.ui with gerrit.enableGwtUi

The gerrit.ui setting is no longer supported. The default UI is always
PolyGerrit and this cannot be changed. It is possible however to make
the GWT UI available by setting gerrit.enableGwtUi; in this case the UI
still defaults to PolyGerrit but allows the user to switch to GWT.

Change-Id: Ifbeca8ea7d1c9c90c7f86e184fc365288129c1dd

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