add notes about jitter to the appropriate place(s)

[critzo]

We recently answered a support request about jitter, and this should be added to the appropriate place(s) on the website.

NDT7 will provide RTTVar (as defined in the kernel here- https://github.com/torvalds/linux/blob/master/include/uapi/linux/tcp.h#L245 ) This measures round trip time variance, which frequently what people mean when they ask for jitter.