Fix a few issues with the lockstat provider man page.

• Address most of the post-commit comments on D11128.[1]
• Reference the man pages for the lock types supported by the provider.
• Add a BUGS section.
• Eliminate some redundancy by describing similar probes in the same paragraph.
• Fix several inaccuracies, particularly in the probe argument descriptions.

Submitted by: wblock [1]
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11293