Add AX.25 T3 timer to be configurable -- Add notes about other AX.25 parameters to documentation

[dranch]

Per our conversation: "A user configurable T3 could be added for completeness. . . . If you think it is important, throw it in the issues tracking so it does not get lost in the clutter."

Also a few reminders just for the documentation side for those users who come from hardware TNCs:

T2 timer: Add the description: " This parameters was removed from the AX.25 specification in v2.2. It is not needed if the protocol stack knows the modem data carrier detect status (which Direwolf does). See the Why-is-9600-only-twice-as-fast-as-1200.pdf document for more explanation.

FRACK: Mention in the User Guide that the max is 15

CHECK: Add this item with the description "Make a periodic check/poll to determine that a connection still exists. Dire wolf does not currently have an option (to configure) this. It is hardcoded to be 300 seconds."

SENDPAC, PACTIME, and CPACTIME: Add these items with the description "These parameters seem to be applicable only to the case where you are talking to a TNC in CONVERS (interactive transmit/receive) mode. Some specified character (typically enter) results in a packet being sent immediately. With both KISS and AGW, applications deal with complete packets, not individual characters so this does not apply to Direwolf's layer."