Document behaviour of sample rate/count settings

[narc0tiq]

As mentioned in #2, it is important to document what happens when you set the sample rate/count such that there aren't enough samples to generate change rates/estimates:

• if the sample rate is too frequent or the sample count is insufficient, the estimate to depletion will be "Insufficient data..." forever. This is expected. However,
• if there are just barely enough samples for the slow signals' update rate (see Correctly handle rarely-updating signals #13), then you occasionally get enough data to generate an estimate, but it sometimes isn't (sometimes you have only two buckets), which resets back to "Insufficient data...".

[narc0tiq]

Closed the wrong one, history has been altered.