(#7773, 7776, 7764) Several help template tweaks

Per UX review of help output, this commit makes several changes to templates and
shared help text:

• Change "unknown" to "undocumented"
• Remove copyright from short help
• Point readers to the man pages (issue 7773)
• Remove examples from short help (issue 7776)
• Remove summary from short help and make it a fallback for description
• Edit common option summaries to fit on a single 80-col line