[3.12] Document that `os.link()` is not available on Emscripten (GH-104822) (GH-107308)

Document that `os.link()` is not available on Emscripten (GH-104822)
(cherry picked from commit 737d1da0746053d515158eac5b115e8bd813f6d3)

Co-authored-by: Roman Yurchak <rth.yurchak@gmail.com>