doc: butc manpage fixes

Add missing -rxbind option.

Change-Id: I25fb205dc058ed345bda4d86bd1d344457432939
Reviewed-on: http://gerrit.openafs.org/10410
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>