Rollup merge of #63921 - RalfJung:file-check, r=alexcrichton
Rollup merge of #63921 - RalfJung:file-check, r=alexcrichton

add link to FileCheck docs

Thanks @gnzlbg for pointing me to those docs.

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