`invalid-decorator` default severity is `warn` but the docs don't say so

[DetachHead]

here it says:

The default severity for diagnostics is error unless otherwise noted.

but in the documentation for invalid-decorator it does not mention that its default severity is warn

not sure what the intended severity is, but i would expect this one to be an error, not a warning, because it's a completely invalid usage of the final decorator which is 100% a mistake.

[rchen152]

We intentionally lowered the severity of invalid-decorator to warn due to #2610. So warn is indeed the intended severity, and the docs are out-of-date. Thanks for the catch!