Document need for Access Database capability

In order for pull-replication to replicate `All-Users` refs, the `Access
Database` capability needs to be enabled for the user performing
replication.

The lack of such capability would prevent All-Users' meta refs from
being replicated.

Document this requirement to make Gerrit admins able to configure
pull-replication correctly.

Bug: Issue 16594
Change-Id: Id5c8ed4cf35b21e725c52d8dee397f12ab3afad1