syscall: document relationship among Errno, errors.Is and os.Err*

- Add doc to syscall.Errno (and syscall.ErrorString for plan9).

- Mention under `syscall` in release notes.

Fixes #33436.

Change-Id: I032ffebaa76ed67eb9d748e7645ca73f26144ea0
Reviewed-on: https://go-review.googlesource.com/c/go/+/191337
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/syscall_windows.go

@@ -77,6 +77,12 @@ func UTF16PtrFromString(s string) (*uint16, error) {
 }
 
 // Errno is the Windows error number.
+//
+// Errno values can be tested against error values from the the os package
+// using errors.Is. For example:
+//
+//	_, _, err := syscall.Syscall(...)
+//	if errors.Is(err, os.ErrNotExist) ...
 type Errno uintptr
 
 func langid(pri, sub uint16) uint32 { return uint32(sub)<<10 | uint32(pri) }