Explain in CellServDB man page that server lines can be omitted

Explain in CellServDB man page that server lines can be omitted

It's possible to use AFSDB records only to locate the VLDB servers but
still list the cell in the client CellServDB so that the client is
aware of it and populates it into dynroot.  Describe doing this in the
man page.

Change-Id: I714cd515dc4b72a6e358bbd8f9332d4ddce5a5fc
Reviewed-on: http://gerrit.openafs.org/1710
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>