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 has no action. Since it's describing a function, not a field, it should describe what the function does, i.e.

```rust
/// Returns the integer part of the number (rounds towards zero).
```