docs: Update manpages for Quassel 0.13.1, cleanup

In brief

• Update manpages to document new options since Quassel 0.13.0 release
  • Add the --ident-listen command line argument.
• Add NOTES section to quasselcore documenting the need for sudo and --configdir
  • Should hopefully reduce cases of running quasselcore as root and breaking permissions
• Clean up formatting according to best practices
  • See man man-pages, https://liw.fi/manpages/
  • Main changes: .PP paragraphs, no blank lines

Postpone merging this until the upcoming Quassel 0.13.1 release has been accepted.