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 golang#33436.

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

doc/go1.13.html

@@ -961,6 +961,13 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       <a href="/pkg/syscall/?GOOS=windows#Chmod"><code>Chmod</code></a> mode on Windows.
     </p>
 
+    <p><!-- CL 191337 -->
+      Values of type <code>Errno</code> can be tested against error values in
+      the <code>os</code> package,
+      like <a href="/pkg/os/#ErrExist"><code>ErrExist</code></a>, using
+      <a href="/pkg/errors/#Is"><code>errors.Is</code></a>.
+    </p>
+
 </dl><!-- syscall -->
 
 <dl id="syscall/js"><dt><a href="/pkg/syscall/js/">syscall/js</a></dt>

src/syscall/syscall_js.go

@@ -44,6 +44,12 @@ const PathMax = 256
 //	if errno != 0 {
 //		err = errno
 //	}
+//
+// 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 (e Errno) Error() string {

src/syscall/syscall_nacl.go

@@ -51,6 +51,12 @@ const PathMax = 256
 //	if errno != 0 {
 //		err = errno
 //	}
+//
+// 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 (e Errno) Error() string {

src/syscall/syscall_plan9.go

@@ -20,6 +20,12 @@ const ImplementsGetwd = true
 const bitSize16 = 2
 
 // ErrorString implements Error's String method by returning itself.
+//
+// ErrorString 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 ErrorString string
 
 func (e ErrorString) Error() string { return string(e) }

src/syscall/syscall_unix.go

@@ -107,6 +107,12 @@ func (m *mmapper) Munmap(data []byte) (err error) {
 //	if errno != 0 {
 //		err = errno
 //	}
+//
+// 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 (e Errno) Error() string {

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) }