More documentation and man page fixes

- Include numbers in man page references
- Use “:manpage:” where appropriate
- Don't escape underscore in verbatim text (e.g. “``always_failover``)
  as it would show up in the generated man page
- Quote whole paths, not just build-time part

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>