Bunch of improvements on explaining `impl Trait`

[earthengine]

The section 10-2 especially the impl Trait part is incorrect or at least misleading.

This work is:

• Being verbosely explained that impl Trait means anonymous type, because many people believe it is just syntax sugar in argument position, and existential in return position. This is misunderstanding of this feature.
• Corrected misleading concepts behind desugaring (expecially, two impl Trait on the same trait means two different types)
• Given better explaination for the single type restriction when use impl Trait in return position, with example codes of alternatives.

[carols10cents]

Hi, thank you for the PR! However, I think a lot of these are details that new Rust users don't need to know right away. The book is very much trying to give just what you need to know to be productive, and other resources like the Reference or the Nomicon or perhaps new resources are better places for the details you've added here. So thank you, but I'm going to close this.