Replace doc mention of periodic timestamp syncing with what actually happens

[truher]

allwpilib/ntcore/doc/networktables4.adoc

Line 83 in 77dfad9

The topic ID of -1 is reserved for timestamp communication. Clients shall periodically (e.g. every few seconds) send, in a manner that minimizes transmission delays, a MessagePack message with ID of -1, a timestamp of 0, and an implementation-selected type and data value (typically int or float 64) containing its (the client's) current local time.

I think what actually happens is, perhaps "client sends periodic messages" and "server ignores messages after the first received".

Is that right?