Fix a few issues with the lockstat provider man page.
- Address most of the post-commit comments on D11128.
- 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 
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11293