Doc: clarify source of jitter in USB1.1, and USB2.0

Even though the jitter due to USB1.1 may be 1ms,
NTP can reduce its effect significantly.  And
USB2.0 reduces this anyway.

Signed-off-by: Sanjeev Gupta <ghane0@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>