Rewrite Git structure guidance in English

Instead of a list of bullet points, explain in plain English how to write a Git commit message, with a little more context as to the reasoning behind it.

This duplicates some of the explanation from the guides we link to, such as the explanation on why to wrap to 72 characters, but also covers reasoning we hadn't documented before, like why you shouldn't end the first line with a full stop.
:eggplant: