improve query system documentation

[jdonszelmann]

Over the last week, I attempted to thoroughly understand rust's query system by implementing a toy one myself. That's not very relevant, but what is, is that I ended up reading the documentation of the query system a bunch of times. I was confused by a few things, and thought I'd clear them up for the next reader. Let me know if there's anything that should be changed.

[jdonszelmann]

In case this is useful, I think I'm mostly adding notes on posts mostly written by you @nikomatsakis. So I guess you'd be the best reviewer for it?