gh-137952: update `csv.Sniffer().has_header()` docs to describe the actual off-by-onish behavior (GH-137953)

* checks 21, not 20
* Say "header" instead of "first row" to disambiguate per review.

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Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Maurycy Pawłowski-Wieroński <maurycy@maurycy.com>