Enhance comment formatting

[topecongiro]

This PR

1. implements recover_missing_comments_in_span(), which is generalized function for recovering missing comments in the given span.
2. use it to recover missing comments.
3. preserve blank line between doc comment and attribute
4. apply some refactoring (e.g. use String::with_capacity() instead of String::new())

Closes #1887.
Closes #1911.

[nrc]

Awesome! Thanks!