nbd: Document timeline of various features

It can be useful to figure out which NBD protocol features are
exposed by a server, as well as what features a client will
take advantage of if available, for a given qemu release.  It's
not always precise to base features on version numbers (thanks
to downstream backports), but any documentation is better than
making users search through git logs themselves.

This patch originally stemmed from a request to document that
pristine 3.0 has a known bug where NBD_OPT_LIST_META_CONTEXT
with 0 queries forgot to advertise an available
"qemu:dirty-bitmap" context, but documenting bugs like this (or
the fact that 3.0 also botched NBD_CMD_CACHE) gets to be too
much details, especially since buggy releases will be less
likely connection targets over time.  Instead, I chose to just
remind users to check stable release branches.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181215135324.152629-3-eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

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