docs: Code128 documentation and example improvements

[chrisp60]

I feel describing the character sets for Code128 may help other users deal with issues relating to its API.

source

128A (Code Set A) – ASCII characters 00 to 95 (0–9, A–Z and control codes), special characters, and FNC 1–4
128B (Code Set B) – ASCII characters 32 to 127 (0–9, A–Z, a–z), special characters, and FNC 1–4
128C (Code Set C) – 00–99 (encodes two digits with a single code point) and FNC1

I would be happy to make a PR with an excerpt added to the docs and possibly a more descriptive error message.

This is definitely not the responsibility of the library to educate people on the character sets, but also I do not think it would hurt to include.

possibly tangentially related to #20.

[buntine]

Thanks, feel free to open a PR for those docs 👍

[buntine]

Merged. Thanks!