outputstream: Clarify docs of flush()

outputstream: Clarify docs of flush()

After questioning the semantics of flush on IRC, it seemed necessary to
clarify what flushing is supposed to do. The Linux man page for fflush()
seemed to cover it perfectly, so I just copied it.

I did not add the "via the underlying write mechanism" part as that in
my opinion is not something subclasses should need to guarantee.