Improve docs for buffer.toString and buffer.copy

Improve documentation for buffer.toString and buffer.copy per issues #8859 and #8857.
@tjfontaine It was made to operate similar to `Array#slice()`.
```javascript
> [1,2,3].slice('foo', 2)
[ 1, 2 ]
> [1,2,3].slice(0, 'foo')
[]
```