doc: Edits to the 'afsd -volume-ttl' manpage

doc: Edits to the 'afsd -volume-ttl' manpage

Make a few misc changes to the text for the new -volume-ttl option:

- Minor grammatical/typo fixes

- Emphasize a little more that the default behavior allows for vldb
  info to be cached _forever_

- Provide some info on the effects of changing this value

- Provide a suggested "typical" value, to give some clue as to what
  should be set here, so a curious user doesn't just set this to the
  first value they see (10 minutes)

Reviewed-on: https://gerrit.openafs.org/12909
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Andrew Deason <adeason@sinenomine.net>
(cherry picked from commit e6c2624249a6ab96053c1d1134aec8e3f6bcee9e)

Change-Id: I781ec2e8b4873093f65d11b5883f8b74ad397cff
Reviewed-on: https://gerrit.openafs.org/12913
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>