doc: document Windows os.ReadDir improvements

doc: document Windows os.ReadDir improvements

For #61422.

Change-Id: Icc1868ef9516766d0f422849866ef8de27bfa4c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/549275
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>