doc: rxdebug manpage fixes

Add missing option -long.

Change-Id: I23e3bc38e6962b7727a2aaa07bab7b0818ebcee0
Reviewed-on: http://gerrit.openafs.org/10406
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>