I would like to say that the latter interpretation is "obviously" correct, except that it clearly wasn't obvious to you.  The fact that it "contains the output stream" I would have thought was pretty clear: if you are writing to it, you are responsible for anything you do to that output stream.  The "must" refers to the same the the RFC "must" refers to: in order to be RFC compliant, you must conform to the RFC.

Perhaps we could clarify that sentence by saying: "Proper adherence to the HTTP protocol must be used when writing to this stream in order to achieve successful interoperation with http clients."  I think that would make it clear that if you don't, you'll be testing the client's response to protocol violations :)