* Docs: Add bz2 usage examples

- Adds an "Examples of usage" section inspired by the one
  found in the gzip docs
- Corrects the descriptions for ``compresslevel`` and ``data``:
    - ``compresslevel`` must be an `int`, not any number.  For
      instance, passing a float will raise ``TypeError``
    - Notes that `data` must be bytes-like