doc: document os.ModeSocket support on Windows

Updates #33357.

Change-Id: I66866475447e03f604202f34ddfbafdc0512cb72
Reviewed-on: https://go-review.googlesource.com/c/go/+/565135
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

doc/next/6-stdlib/99-minor/os/33357.md

@@ -0,0 +1,3 @@
+The [`os.Stat`](/os#Stat) function now sets the [`os.ModeSocket`](/os#ModeSocket)
+bit for files that are Unix sockets on Windows. These files are identified by
+having a reparse tag set to `IO_REPARSE_TAG_AF_UNIX`.