I don't believe there is any specific document on how that function works.
The changed by function is simply that, when ever the channel changes by the amount you have set the output will be true. Below is a bit of logging from a test I did. The blue channel is my input and the orange is the output. The condition was changed by 1.
If the input changes by the set value within the calculation period (0.04 seconds) then the output will become true. It will remain true for 1 cycle and then go false unless the channel has changed by the set value within that cycle.
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