Clarify text for PUSH_PROMISE

I realize this is somewhat redundant given section 5.1.1, so I don't mind if this pull is rejected. The main reason I think this might be useful is since there are two different stream identifiers involved in PUSH_PROMISE: the promised stream id and the stream identifier in the common header. The extra sentence just makes it explicit how PUSH_PROMISE is linked to an earlier request. It also puts the following sentence in better context.