Guidelines for referring to rustc lints

[ehuss]

I think the authoring guide should have some guidelines on referring to rustc lints.

Generally we prefer to avoid referring to them, but in several cases it is a disservice to not mention them. It would be helpful to have guidance on how to balance those kinds of references.

Some examples of lints that are currently referred to:

• unsafe_op_in_unsafe_fn
• ffi_unwind_calls
• unused_variables
• unused_must_use
• unfulfilled_lint_expectations
• overflowing_literals

There's other cases where it currently doesn't refer to the specific lint, for example the #[deprecated] attribute, which arguably would benefit for a link to it.

As part of the attribute rework, we have a bunch of notes about rustc linting about certain things (like unused attributes, or future-incompatible warnings), but they don't refer to the lint by name.