A36 update: edit for clarity, fix some inconsistencies

[murgatroid99]

From my perspective working on implementing this design, there are three major parts:

• How the server interacts with the xDS API.
• How the server behaves as a result of the configurations from xDS.
• The changes to xDS resource validation rules.

I rearranged some of the information to structure the design that way. I also made a few other changes that improve clarity, in my opinion:

• Removed a couple of statements that contradict other gRFCs
• Decreased focus on some potential use cases that have not yet materialized
• Added a couple of callouts using GitHub markdown's recent alerts feature.
• Added more subheadings to break up longer sections.