Doc comments on `use` declarations are limited to 5 characters less than `max_width`

If wrap_comments is true, doc comments on use declarations are wrapped to 5 characters less than max_width (or comment_width, if it is less than max_width - 5). This makes it impossible to wrap such comments to the same value as max_width.

Example:

test.rs:

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// aa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
pub use u32;

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// aa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
pub struct X;

After running rustfmt --config wrap_comments=true,comment_width=79,max_width=79 test.rs:

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a a a a aa a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a a a a a a a a a a a a a a
pub use u32;

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a aa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a a a a a a a a a
pub struct X;

The comment on the use declaration is wrapped to 74 characters, exactly 5 less than 79. This five-character difference persists across different values of comment_width and max_width, except if comment_width is more than 5 characters less than max_width, both doc comments will be limited to that value.

Start with the supplied test.rs example and run rustfmt --config wrap_comments=true,comment_width=79,max_width=79 test.rs to reproduce the differing wrapping for the use and struct doc comments. Trace the wrapping behavior for use declarations, then verify that both comments can wrap to the configured width and that the command output no longer has the five-character discrepancy.