docs(dateFilter): fix docs to match implementation for week no formatting

The existing documentation claims that dateFilter determines week no
according to the ISO8601 standard, but this is not the case as illustrated
by tests in this PR. More specifically, the implementation deviates from
ISO8601 in 2 important aspects:
- impl assumes Sun to be the first day of a week, ISO8601 mandates Mon
- impl allows weeks 0 (for years starting on Fri, Sat) while ISO8601
would mark them as a week 52/53 of a previous year.

Fixes #10314
Closes #10313

This PR is an alternative approach to the one proposed in #10313. Merging #10313 would introduce a breaking change without assuring full compliance with ISO-8601
We are not really assuming anything, I think it is better phrased as 
`Week of the year (00-53). Week 01 is the week with the first Thursday of the year`