DOC-7629 Update bucket-edit eviction policy documentation

It's not possible to change the eviction policy for ephemeral buckets,
however, the documentation incorrectly stated that the
'--bucket-eviction-policy' flag was valid for both Couchbase and
Ephemeral buckets.

Change-Id: I22bfed98f88ca69932bff241e29703e7b3ca7be2
Reviewed-on: http://review.couchbase.org/c/couchbase-cli/+/139741
Well-Formed: Build Bot <build@couchbase.com>
Reviewed-by: Carlos Gonzalez <carlos.gonzalez@couchbase.com>
Tested-by: James Lee <james.lee@couchbase.com>

Author: jamesl33