Add capturing and precise capturing rules

[traviscross]

The Reference didn't include any description of capturing or capturing behavior for impl Trait opaque types. Let's describe briefly what capturing is and what the currently-stable automatic capturing rules are. Then let's describe the syntax and behavior of RFC 3617 precise capturing.

Precise capturing is currently undergoing stabilization in:

• https://github.com/rust-lang/rust/pull/127672

Tracking issue:

• https://github.com/rust-lang/rust/issues/123432

cc @compiler-errors @ehuss