[RFC 231] Capture mode and closure trait rules

[crlf0710]

I always have to open RFC 231 to check when i need to explain these rules to others
So i copied these text (with minimal editorial changes) from RFC 231 to The Rust Reference so people can read them more easily.

[Havvy]

I'm too tired to properly review this, but this is definitely the kind of improvements I like to see. 😄

[alercah]

Thanks! There's a fair bit of this content already on the closure types page, can you please make sure that it isn't redundant?

[crlf0710]

@alercah Oops, i somehow overlooked those text. So I replaced my patch with just adding a heading there and linked to it. How does that look?

[alercah]

Looks good! I had some WIP on fixing up this section to be more consistent overall with the types section, but I lost the computer it was on, and this is a definite improvment! If you want to try to work more in this space and get the two meshing better, go for it. 👍

Thanks for the PR! ❤️