Unclear explanation of fn_to_numeric_cast_with_truncation

[dtolnay]

https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#fn_to_numeric_cast_with_truncation

What it does

Checks for casts of a function pointer to a numeric type not enough to store address.

Why is this bad

Casting a function pointer to not eligable type could truncate the address value.

"Eligable" looks like it was intended to be "eligible", but I find "eligible type" vs "not eligible type" not a clear way to identify types that are wide enough to hold the address vs not wide enough.

I think this lint would be sufficiently justified by saying:

- Checks for casts of a function pointer to a numeric type not enough to store address.
+ Checks for casts of a function pointer to a numeric type not wide enough to store address.

- Casting a function pointer to not eligable type could truncate the address value.
+ Such a cast discards some bits of the function's address. If this is intended, it would
+ be more clearly expressed by casting to usize first, then casting the usize to the
+ intended type (with a comment) to perform the truncation.

Mentioning @VKlayd who worked on this lint in #2814.

[phansch]

This was fixed as part of #3251 (fn_to_numeric_cast and fn_to_numeric_cast_with_truncation)