Documentation changes (for issue #159).

This contains fixes for most of issue 159 (the parts I could actually
do).

- Fixed consistencies between `--help` and `--man`
- Clarified default for `-H` option
- Change all occurrences of REGEX in man to PATTERN
- Added option/documentation for `--without-filename`
- Changed to `--[no]group` style throughout man, except where the positive
  option requred an argument (e.g. `--pager=program` and `--nopager`, and not
  `--[no]pager=program`)