Add note in 6.2.5 add and erase about strong relation to 6.2.2.4 rtt presentation

There has been questions about how to present text after erasure. Erasure is specified in 6.2.5, while info on grouping of text and placement in approximal time order is in 6.2.2.4. A note about this interrelation is proposed here, as hinted in #412 (closed)

So, new note proposed in 6.2.5:

NOTE 7: See clause 6.2.2.4 of the present document about presentation aspects of adding and erasing text.