Add more description to c_str::unwrap().

It's unclear what you are supposed to do with this memory.
Let's make that more clear.
> Unwraps the wrapped `*libc::c_char` from the `CString` wrapper.
> 
> The original `CString` object is destructed after this method is called,
> and if the underlying pointer was previously allocated care must be taken
> to ensure that it is deallocated.

Something like that?