doc: clarify the fs wscell manpage

doc: clarify the fs wscell manpage

What's displayed by fs wscell is not necessarily the current content
of ThisCell, but that at the time of starting the client. Say so.

FIXES 133339

Reviewed-on: https://gerrit.openafs.org/12537
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit bd15a5f56fde98983464acf5fd4cdd731d206d9f)

Change-Id: I47d7b92488b1166934a1704765c0f1e914a178a8
Reviewed-on: https://gerrit.openafs.org/12559
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>