Misleading documentation on into_inner methods of I/O writers

[mzabaluev]

The documentation of into_inner method on BufWriter, BufStream, and LineWriter says that the internal buffer is 'flushed' before the underlying writer is returned. This may confuse the reader into thinking that flush is called on the underlying writer. The current implementation does not perform such deep flushing, and it really shouldn't. Rather, the documentation should make clear that the buffered data are only written out, so it's the responsibility of the consumer of the returned writer to eventually flush it.