Added clarification about when calculation is performed

[sadiqkhoja]

Related: https://github.com/XLSForm/pyxform/issues/546

Background:

Consider a form with two textboxes, second one is calculated. Enketo and Collect both let user to change the value in the second textbox but Collect recalculates the field at the time of submission and Enketo keeps the user entered value.

I can use xforms-value-changed to achieve the same behavior in Collect i.e. to recalculate only when first textbox is edited.

We need specification to mention when client is expected to perform calculation.