known-bug directive syntax is not documented

[lolbinarycat]

the known-bug directive requires an argument, which is a comma-separated list of issue IDs.

this is not clearly documented.

https://rustc-dev-guide.rust-lang.org/tests/ui.html#known-bugs

[lolbinarycat]

also, shouldn't known-bug imply run-fail or invert the expected outcome? if not, that should probably be noted.

[tshepang]

there is also check-fail and build-fail, so I don't think run-fail would be specailly applicable

also, I see a number of known-bug tests marked as check-pass, which is for cases where they are expected to fail when the bug gets fixed