ddb(4): misc updates

• Describe optional 'addr' argument to many show commands
• Remove obsolete commands (show cbstat)
• 'show jails' was renamed to 'show prison'
• Remove superfluous commentary about sleepqueues
• Fix an xref to gdb(4)
• Fix issues reported by mandoc -Tlint
• Plus a couple other inaccuracies/inconsistencies

Reviewed by: pauamma, markj, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation (in part)
Differential Revision: https://reviews.freebsd.org/D37332