Stabilization docs for RFC 2300 - self_in_typedefs

[Centril]

RFC: rust-lang/rfcs#2300
Stabilization was done (rust-lang/rust#56366) and it will be in 1.32.
cc rust-lang/rust#49303

[Centril]

@alexreg suggested that this was already done in the reference... but it would be good to double check...

[ehuss]

Unless I'm missing something, I don't think it is documented. It is explained for traits and impls, but I don't think there is much detail besides that. The section on Self should probably be updated to include a link to something that describes self_in_typedefs. I don't know where that should live (types chapter maybe?).

[alexreg]

It's already in the grammar.

[Centril]

An idea: in the recursive types section: https://doc.rust-lang.org/nightly/reference/types.html#recursive-types we could illustrate the use of Self... Otherwise @alexreg is right that it is in the type expression grammar.

[alexreg]

@Centril Yep, that could work.