[RFC] Nvim specific docs: Misc. improvements

Besides the items mentioned in the commit message, most of the rewritten portions were done for clarity and consistency, but much of that is of course subjective so feel free to criticize.
Looks like "goal" is repeated. Anyways, it bugs me that we have an entire chapter that starts with the word "A" :) How about this as a first sentence:

> Extensibility is a primary goal of Nvim. Any programming language may be used to
> extend nvim without changes to nvim itself. This is achieved...