doc: Add 'fs discon' manpage

Change-Id: I67c3bf50bfb7f4f7c0eb46cd011cc8ae68ec5302
Reviewed-on: http://gerrit.openafs.org/10415
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>