Policy: Referring to rustc lints

[ehuss]

Generally the Reference has avoided ever mentioning rustc lints, but unfortunately there are a few lints that are really relevant and important to certain sections of the language.

• ehuss's preference: Have a policy against referring to them except in extreme situations where an Editor's note provides some very useful context. For example unsafe_op_in_unsafe_fn, and in particular its relationship to the new Edition.