Editorial: Highlight diff in B.3.4 compared to the original semantics

[nicolo-ribaudo]

This is also done in B.3.1, B.3.2.4 and B.3.2.5, and it makes it easier to understand how Annex B is modifying the original semantics.

[michaelficarra]

@jmdyck Could we incorporate this into #2952 using the flagging approach from that PR?

[jmdyck]

PR #2952 already incorporates it.

[michaelficarra]

@jmdyck Oh, I looked but must have missed it because of the big difference in line numbers.

Okay, then I am going to close this in favour of #2952, since we already decided we will take that approach.