Add PBKEYLEN negotiation table to the documentation.

[maxsharabayko]

Add PBKEYLEN negotiation table to the documentation.

If both parties set PBKEYLEN and the value on both sides is different, this is considered a conflict. It is resolved by the Initiator party, which takes its own value if the SRTO_SENDER option is set to true. Otherwise, it takes the value from the Responder party. The assignment of Initiator-Responder roles matches the Caller-Listener layout. In the case of Rendezvous this assignment depends on the result of the cookie comparison.

* - any.

Initiator		Responder		Result
SRTO_PBKEYLEN	SRTO_SENDER	SRTO_PBKEYLEN	SRTO_SENDER
0	*	0	*	AES-128
0	*	AES-128	*	AES-128
0	*	AES-192	*	AES-192
0	*	AES-256	*	AES-256
AES-128	*	0	i	AES-128
AES-128	*	AES-128	g	AES-128
AES-128	0	AES-192	n	AES-192
AES-128	0	AES-256	o	AES-256
AES-128	1	AES-192	r	AES-128
AES-128	1	AES-256	e	AES-128
AES-192	*	0	d	AES-192
AES-192	0	AES-128	*	AES-128
AES-192	*	AES-192	*	AES-192
AES-192	0	AES-256	*	AES-256
AES-192	1	AES-128	*	AES-192
AES-192	1	AES-256	*	AES-192
AES-256	*	0	*	AES-256
AES-256	0	AES-128	*	AES-128
AES-256	0	AES-192	*	AES-192
AES-256	*	AES-256	*	AES-256
AES-256	1	AES-128	*	AES-256
AES-256	1	AES-192	*	AES-256