docs: mention maintenance branches

docs: mention maintenance branches

Mitre tried to assign us two separate CVEs for the fix for
https://bugzilla.redhat.com/show_bug.cgi?id=1047577, on the
grounds that the fixes were separated by more than an hour
and thus triggered different hourly snapshots.  But we
explicitly do NOT want to treat transient security bugs as
CVEs if they can only be triggered by patches in libvirt.git
but where the problem is cleaned up before a formal release.

Meanwhile, I noticed that while our wiki mentioned maintenance
branches and releases, our formal documentation did not.

* docs/downloads.html.in: Contrast hourly snapshots with
maintenance branches.

Signed-off-by: Eric Blake <eblake@redhat.com>