Reindex Gerrit after project move/rename

* doc/source/gerrit.rst: When project names change, Gerrit's lucene
indexes must be rebuilt while the service is still offline. If this
is not done, searches on the new project names will not find
existing changes and Zuul will ignore some events for them.

Change-Id: I99cd657298e05f7c213cf770e92d06016043f547
Merge "Reindex Gerrit after project move/rename"