Document the global rustfmt.toml feature.
Document the global rustfmt.toml feature.

graydon/rust,graydon/rust,aidancully/rust,SingingTree/rustfmt,graydon/rust,aidancully/rust,SingingTree/rustfmt,aidancully/rust,graydon/rust,rust-lang/rustfmt,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,rust-lang/rustfmt,graydon/rust