doc: recommend cleanup steps in "vos convertROtoRW" man page

vos convertROtoRW leaves the older RW copy on the original fileserver,
although it is no longer in the VLDB. Provide the user with some hints
regarding clean up.

Reviewed-on: http://gerrit.openafs.org/9408
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: Ken Dreyer <ktdreyer@ktdreyer.com>
(cherry picked from commit 279345c231d0a2d9f6e8c2f76a5347bafd40e70b)

Change-Id: Id273ab86bfe68f89ef629f0c4d839882852e09ab
Reviewed-on: http://gerrit.openafs.org/11126
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>