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.

Change-Id: I5f6fcf7d5a516b59438d84e60f163a567d3a64fd
Reviewed-on: http://gerrit.openafs.org/9408
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: Ken Dreyer <ktdreyer@ktdreyer.com>