Update most documentation for NoteDb

Remove outdated references to ReviewDb. Where ambiguous terms like
"database" are used, make sure they are still technically accurate, or
else remove them.

After this change, all documentation should be up to date with respect
to the database backend, with the exception of note-db.txt itself.

Change-Id: I7134087dc56070788e81ada6ac504b3c8fe84c4c
diff --git a/Documentation/cmd-index.txt b/Documentation/cmd-index.txt
index e999218..edb54b5 100644
--- a/Documentation/cmd-index.txt
+++ b/Documentation/cmd-index.txt
@@ -71,7 +71,7 @@
 	List projects visible to the caller.
 
 link:cmd-query.html[gerrit query]::
-	Query the change database.
+	Query the change search index.
 
 'gerrit receive-pack'::
 	'Deprecated alias for `git receive-pack`.'