Improve documentation on `projection`

[FichteFoll]

via https://github.com/SublimeText/sublime_lib/pull/166#discussion_r637393306

Since we use selectors twice in sublime_lib, I really believe this explanation should go into its own section that we then reference. Kind of how the official docs have a types section that they link to for such special types.

Because projection isn't a function directly used by the user of this API and they can't change the first parameter, I believe it makes more sense to explain the effects of the second parameter only. I just don't think think the technical explanation is useful.