qapi: bitmap-merge: document name change

We named these using underscores instead of the preferred dash,
document this nearby so we cannot possibly forget to rectify this
when we remove the 'x-' prefixes when the feature becomes stable.

We do not implement the change ahead of time to avoid more work
for libvirt to do in order to figure out how to use the beta version
of the API needlessly.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20180919190934.16284-1-jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: typo fix]
Signed-off-by: Eric Blake <eblake@redhat.com>