doc: limitations of addsite on different partitions

doc: limitations of addsite on different partitions

A user on the openafs-info mailing list noted that the Admin Guide is
unclear about creating read-only replicas on different partitions on
the same fileserver. Clarify the rules here.

Reviewed-on: http://gerrit.openafs.org/5745
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit c59c07132ef17589ad3466ded418c1370539b7af)

Change-Id: I68898c21e2c1aa7d6828b475dff4c1cb6d14327c
Reviewed-on: http://gerrit.openafs.org/6317
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>