[3.12] gh-108202: Document calendar.TextCalendar formatting helpers (GH-127608) (#128441)

Document the following TextCalendar methods:
- formatday()
- formatmonthname()
- formatweekday()
- formatweekheader()

(cherry picked from commit bb73426cafb78691b249ffa50f1872ab6f899d4a)

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