Identifying streams more precisely

Two issues were raised about the use of the words "existing" and "current" to refer to streams.  These were a little squishy, so I tightened the language a little.  The text no longer uses either word to refer to streams, instead using stream states to describe where the relevant text applies and favouring "identified" as a fallback.
My concern is that this not obscure the fact that it also changes the window size for all idle streams on which you might be able to send data in the future.  Hopefully obvious, but let's try not to confuse an overly-literal reader.