W-15256317-Add note about cron expressions

[mtransier]

I stumbled upon the issue that my cron expression was not accepted and I could not find out why until I looked it up in the source code. It is mandatory to use the '?' value for one of the two day fields.

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[dmerlob]

• Hi, @mtransier. Thanks for your contribution! We have put it in our queue to evaluate.

[dmerlob]

This request is addressed in the PR #2798.