Improve documentation of rounding functions

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.
This comment is describing the current behavior, but as documented in #15826 the current behavior is wrong. It should document both the expected behavior and the current b roken behavior. Perhaps something like

```rust
/// Rounds to the nearest integer. Rounds half-way cases away from zero.
///
/// Note: This function is currently broken and always rounds away from zero.
```

I'd suggest maybe even adding something like `**[Broken]**` onto the summary line, but we have no precedent right now for doing that sort of thing so probably best to avoid it.