Align week year explanation with other docs

[HedwigAR]

Hi!
We got a report that the current explanation of week years is unclear. This PR aligns the explanation with the one in parse-and-format-date-function-calls.md
I checked to see if the whole doc could be aligned but since they do differ I chose to just replace this explanation.

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ OlufunkeMoronfolu
❌ HedwigJDoets
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