Reformat doc comments to fit in 80 columns.

Created by: jimblandy

No hard feelings if this is not what the maintainers like! But the Rust style guide says:

Source lines which are entirely a comment should be limited to 80 characters in length (including comment sigils, but excluding indentation) or the maximum width of the line (including comment sigils and indentation), whichever is smaller:

I applied an 80 column limit to all doc comments, regardless of their indentation. I don't recall anything that was indented more than four columns, so this should make little difference.