Improve documentation of rounding functions

[Sawyer47]

Important note: I documented current behaviour even though it may not be correct (see issue #15826). round() function for rationals does different rounding than round() from Float trait.

[lilyball]

As long as you're updating these, it would be nice to change them to be more consistent with the prevailing style, i.e.

/// Rounds towards minus infinity.

(note: according to http://aturon.github.io/style/comments.html the summary line is supposed to end in punctuation, which I think is not actually how most of our comments work right now, but better to match the documented style).

[Sawyer47]

Thanks for comments. I've changed other comments too, for consistency.