gh-131885: Update csv.rst to describe that `csv.reader` and `csv.wri…

[adamtheturtle]

…ter` have some positional-only arguments

import csv

with open("example.csv", "w", newline='') as csvfile:
    csv.writer(csvfile=csvfile)  # TypeError:  expected at least 1 argument, got 0

import csv

csv.reader(csvfile="a")  # TypeError:  expected at least 1 argument, got 0

• Issue: gh-131885

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