Alembic does lots of new things. Provide docs for how to use this. We
also improve upgrade docs slightly, removing references to ancient
reviews that are no longer really helpful as well as calling out our N
-> N+1 constraint.
Change-Id: I3760b82ce3bd71aa0a760d7137d69dfa3f29dc1d
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
