Document // error-pattern

[jyn514]

Example: https://github.com/rust-lang/rust/pull/76143/files#diff-23706e664a2a576e6e031e0be43b04ff0f1916b93c2358b518d67e53aa92fbc3L1

It's used when you don't have a span available in the error but still want to test the output.

Originally posted by @jyn514 in rust-lang/rust#79621 (comment)