doc: document the finish event
doc: document the finish event

Since the stream implementer is not expected to overwrite
.end() the finish event is necessary in order to know when
no more data can be written

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node