lint emitting updating

if you don't run cargo uitest (and maybe cargo test) before tests/ui/update-all-references.sh the script won't do anything.

It would be nice if this could be made more clear. Got me really confused why cargo run and cargo build didn't trigger a change for the script.

[flip1995]

https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md#testing

This chapter would need to be updated. I think adding a note to the sentence mentioning the script would be enough.