[3.12] gh-121196: Document `dict.fromkeys` params as pos-only (GH-121197) (#121243)

gh-121196: Document `dict.fromkeys` params as pos-only (GH-121197)
(cherry picked from commit 1dc9a4f6b20148fd4ef2eb2800a6c65224828181)

Co-authored-by: sobolevn <mail@sobolevn.me>