Maybe document emplacement syntax `<-`

[ehuss]

I think the <- token might need to be included in tokens.md because it is accepted as a token in macros.

Additionally, if x<-1 {} is not valid syntax. It requires a space before -. See rust-lang/rust#51883 That should be specified somewhere.

[ehuss]

For context, there was an attempt to remove the token in rust-lang/rust#60803, but that appears to have been backed out.