add note to 6.2.1.3

The note makes 6.2.1.3 sound like it is all about emergency -- (though it actually doesnt say that exactly). also it is unclear what "modifications during the call" refers to. Also - one might infer that it had to be a single action for calls with RTT when it was multiple actions for a voice only call already. Again it doesnt say that - so we can clarify in a note but it is a bit ambiguous.

Suggest just adding a note 2 to explain

6.2.1.3           Single user operations

Where ICT provides real-time bidirectional voice communication, and supports RTT,
the ICT shall allow single user operations to act on all media during establishment modifications during the communication and disconnections.

NOTE 1:    This requirement is of great importance to ensure that a callback resulting from an emergency communication can achieve the same or better level of accessibility as the original emergency call.

NOTE 2: Modification includes transfer, hold, and other changes to calls or participants. Accoring to this revision, modification of the call with RTT included would be the same as with a voice only call.