doc: add linked cells description to man pages

The man pages previously described linking DCE cells to AFS cells.
OpenAFS and YFS also allow linking between two AFS cells. Update the
description of linked cells in CellServDB(5), aklog(1), and
fs_newcell(1) to refer to AFS instead of DCE.

Add a linked cell example to the CellServDB man page with an
explanation.

Change-Id: Ic9b1c643861b7307c09fcc5a1775f4abf4cb4155
Reviewed-on: http://gerrit.openafs.org/10342
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>