docs: spelling fixes

Here's a bunch of docs-patches that correct minor spelling errors and capitalization.

A few patches change some terms from UK English to US English. The main point is to make things consistent. The reason US English was preferred, is that it seems to be the dominant language used in our docs.

This is far from all spelling mistakes, but it's what I've found so far. My longer term plan is to be able to automatically spell-check the docs, so new problems don't creep in. This helps a bit, as it keeps the size of the custom word list I need to maintain down in size.