RAFT: Add clarifying note for cluster/leave operation.

RAFT: Add clarifying note for cluster/leave operation.

We had a user express confusion about the state of a cluster after using
cluster/leave. The user had a three server cluster and used
cluster/leave to remove two servers from the cluster. The user expected
that the single server left would not function since the quorum of two
servers for a three server cluster was not met.

In actuality, cluster/leave removes the server from the cluster and
alters the cluster size in the process. Thus the single remaining server
continued to function since quorum was reached.

This documentation change makes it a bit more explicit that
cluster/leave alters the size of the cluster and cites the three server
down to one server case as an example.

Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=1798158
Acked-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>