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Remove outdated references to ReviewDb.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

Documentation/cmd-ls-user-refs.txt

diff --git a/Documentation/cmd-ls-user-refs.txt b/Documentation/cmd-ls-user-refs.txt
index cba7d1b..0363f60 100644
--- a/Documentation/cmd-ls-user-refs.txt
+++ b/Documentation/cmd-ls-user-refs.txt
@@ -32,8 +32,8 @@
 --user::
 -u::
 	Required; User for which the visible refs should be listed. Gerrit
-	will query the database to find matching users, so the
-	full identity/name does not need to be specified.
+	will query the index to find matching users, so the full
+	identity/name does not need to be specified.
 
 --only-refs-heads::
 	Only list the refs found under refs/heads/*