Environment documentation

Clarify how r10k works with dynamic environments and how to test using an environment.
Correct capitalization of r10k in a few places.
Take 2 on describing this process. I think including the Puppetfile and its expansion is important, and I've changed the wording to say that branches are only kept in sync when r10k is run again. You have the best understanding of how it works, so if that's not an improvement in describing then I'll just revert this section so the other updates can go in and this can be reviewed later (I'm now seeing that this should have been a separate PR, sorry!)