Rollup merge of #33815 - carols10cents:trait-documentation-clarifications, r=steveklabnik

Rollup merge of #33815 - carols10cents:trait-documentation-clarifications, r=steveklabnik

Trait documentation clarifications

Hi! I've felt a bit of friction lately in figuring out how to write custom implementations of the `derive`able traits, so I decided to add to the docs :)

The docs for `Copy` are already excellent-- clear, useful sections that I only reordered a bit-- they're now:

* General explanation
* When can my type be `Copy`?
* When can my type _not_ be `Copy`?
* When should my type be `Copy`?
* Derivable
* How can I implement `Copy`?

I didn't add all these sections for all the traits, but I did make sure all the derivable traits had a consistent "Derivable" section that explained what the derived implementation does and a "How can I implement" section that has an example.

Please check me for correctness-- I tried to do research to make sure I was saying accurate things but I'm still learning! ❤️ I'd also love suggestions on information to add that is still missing-- I think these traits are important and deserve to have awesome docs!