Documentation improvement on buffer.copy

Per https://github.com/joyent/node/issues/8857.. a slight documentation improvement discussing overlapping buffers...

Per https://github.com/joyent/node/issues/8859 .. slight documentation improvement discussing buffer.toString
What in your mind is the difference between unspecified and `undefined`?
```
> function f(i) { console.log('i=', i) }
> f()
i= undefined
```

What you are describing is a general convention of the node API, I don't think it decreases the quality of the docs.