firewall: Improve the comments (documentation) (#21862)

* Fix the FW names

FW_REFUSE was removed and nixos-fw-input was renamed to nixos-fw.

* Update the comment (documentation) at the top

Order the chains of the main table alphabetically (like in the rest of
the file) and add nixos-fw-rpfilter (from the raw table) and nixos-drop
(used while reloading the firewall).

* Refactor the module (mainly comments)

- Move some attributes to the top for better visibility (that should
  hopefully make it easier to read and understand this module without
  jumping around too much).
- Add some missing examples and improve some descriptions.
- Reorder the mkOption attributes for consistency.
- Wrap lines at 72 characters.
- Use two spaces between sentences.