LOW and HIGH terms might be confusing in SGB docs

"LOW" and "HIGH" are used to refer to signals, but a software developer may not have the necessary background to understand what those mean. They should be at least explained in an info box at the top (as per [the style guide](https://github.com/gbdev/pandocs/wiki/Document-Style#1-conventions-documentation)), and maybe replaced throughout the document where it makes sense to do so.