doc: Add 'fs precache' manpage

Change-Id: Ib4101fdb37cf91a547974216425e4a005392d988
Reviewed-on: http://gerrit.openafs.org/10413
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>