ministat: Clean up usage message, manual page, and chameleon

In the manual page:

• Fix synopsis and use better argument names
• Improve description of flags.
• In EXAMPLES, list the contents of iguana and chameleon files.
• Use Rs to reference The Cartoon Guide to Statistics
• Fix misc indentation and style issues
• Tag SPDX

MFC after: 1 week
Reviewed by: bcr, ziaee
Differential Revision: https://reviews.freebsd.org/D53710