Display original pattern in rustdoc.
Display original pattern in rustdoc.

This commit displays the original pattern in generated documentation for
async functions rather than the synthesized pattern.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust