nixos/weechat: cleanup module, add module documentation

This adds several improvements the previously introduced
`services.weechat` module:

* Dropped `services.weechat.init` as the initialization script can now
  be done on package-level since 2af41719bc using the `configure`
  function.

* Added `sessionName` option to explicitly configure a name for the
  `screen` session (by default: weechat-screen).

* Added `binary` option to configure the binary name (e.g.
  `weechat-headless`).

* Added docs regarding `screen` session and `weechat.service`.