Tracking issue for wasm-unknown-unknown

[carols10cents]

Not sure about Rust issue, maybe rust-lang/rust#44839 ?

Places that need to be updated:

• The nightly version at the top: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/platform-and-target-support/webassembly-support.md#L5
• This spot: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/platform-and-target-support/webassembly-support.md#L24-L28

However, in the meantime, Rust has also grown its own support, independent from Emscripten. This is known as "the unknown target", because instead of wasm32-unknown-emscripten, it's wasm32-unknown-unknown. This will be the preferred target to use once it's ready, but for now, it's really only well-supported in nightly.

[ehuss]

Closing per #232, this section has been removed. Indeed there isn't great documentation for targets. Ideally I think that should live in the rustc book, and is something that has been on my todo list to get started for a while.