net: clarify maxListenerBacklog windows implementation

The previous TODO comments were somewhat ambiguous. This aims to
provide a clearer understanding of the behavior on Windows.

Windows does not offer a way to peek at the current backlog length, this
is explicitly stated in the winapi for `listen`.

When set to `syscall.SOMAXCONN`, the OS dynamically adjusts the
backlog to a maximum reasonable value. It goes as far as the dotnet
runtime itself introducing a new version of `listen` that does not accept a
backlog parameter to help eliminate the confusion when comparing the
behavior with UNIXes.

The docs also mention that `SOMAXCONN_HINT(N)` can be used, and that
it clips the final computed value between (200, 65535), which suggests
windows might use a `uint16` to back this number. Either way it does not
matter since windows will adjust this value anyway, so I removed the
wrapping TODO as well.

See https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-listen

Change-Id: I7b2e7cb547467c4bfc572ef0477a58de8c772521
GitHub-Last-Rev: 34e74abffe
GitHub-Pull-Request: golang/go#63549
Reviewed-on: https://go-review.googlesource.com/c/go/+/535475
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/sock_windows.go

@@ -11,8 +11,9 @@ import (
 )
 
 func maxListenerBacklog() int {
-	// TODO: Implement this
-	// NOTE: Never return a number bigger than 1<<16 - 1. See issue 5030.
+	// When the socket backlog is SOMAXCONN, Windows will set the backlog to
+	// "a reasonable maximum value".
+	// See: https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-listen
 	return syscall.SOMAXCONN
 }