round(x, ::RoundingMode) is not documented

The new `RoundingMode`s are not documented.

Also, the documentation for `round` is no longer accurate for 0.4...?

```
   "round(x)" returns the nearest integral value of the same type as
   "x" to "x", breaking ties by rounding away from zero.
```

On 0.4-dev:

``` jl
julia> round(0.5)
0.0
```

On release-0.3:

``` jl
julia> round(0.5)
1.0
```
