Expand description of limitations in submodule subscription
Expand description of limitations in submodule subscription

Document that adding a subscription will not cause any change in
the submodule revision until changes are merged on the submodule.

Bug: Issue 4451
Change-Id: I67122d5df3951aac8cb703bc24f17b37f7b6b662

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