Review and improve "Acknowledgement and Lost Packet Handling" section

[mbakholdina]

Review and improve "Acknowledgement and Lost Packet Handling" section:

• Introduce loss-triggered and periodic NAK reports terms.
• The difference between 2 NAK packets in "NAK (Negative Acknowledgement or Loss Report)" section.
• The difference between full, small, and light ACKs.
• The following abstract is now in both "Packet Acknowledgement (ACKs, ACKACKs)" and "Too-Late Packet Drop" sections. It better fits "Too-Late Packet Drop" section, however we should tell about fake ACKs in the first section too.

When a receiver encounters the situation where the next packet to be played was not
successfully received from the sender, it will "skip" this packet (see {{too-late-packet-drop}})
and send a fake ACK. To the sender, this fake ACK is a real ACK, and so it just behaves as if the packet had been received.
This facilitates the synchronization between SRT sender and receiver. The fact that a packet was
skipped remains unknown by the sender. Skipped packets are recorded in the statistics on the
SRT receiver.