Lots of docs on salt fileserver environments

This adds a detailed explanation of how the top file is compiled, which clears up both #4409 and #6102. It also adds a "step 4" to the states walkthrough, giving an in-depth explanation of how the salt fileserver combines files from multiple root directories in a single environment, and how this path inheritance can be used to promote states from dev -> QA -> prod (satisfying #6284).