Emit a warning when a codeblock is using "compile-fail" instead of "compile_fail"

[GuillaumeGomez]

I just found out that a lot of error code explanations were using "compile-fail" instead of "compile_fail". I'm fixing this issue as part of something a bit bigger, however I assume this error is pretty common in rust and that rustdoc should warn people about it.

The big issue here is that since it's not a known tag, rustdoc doesn't recognize the codeblock as a rust one and therefore doesn't test it, which is pretty bad.