doc: Add 'fs nukenfscreds' manpage

Change-Id: I51c6cd56f463e10ab51adf20a0cf8505e3361326
Reviewed-on: http://gerrit.openafs.org/10414
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>