doc, api: add syscall.SysProcAttr.AmbientCaps change to 1.9 notes, API

Updates #20587

Change-Id: I08377281270631ee08cd05ba835aa698ca23fa56
Reviewed-on: https://go-review.googlesource.com/47092
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

api/go1.9.txt

@@ -131,11 +131,17 @@ pkg syscall (freebsd-arm-cgo), type Credential struct, NoSetGroups bool
 pkg syscall (freebsd-arm), func Pipe2([]int, int) error
 pkg syscall (freebsd-arm), type Credential struct, NoSetGroups bool
 pkg syscall (linux-386-cgo), type Credential struct, NoSetGroups bool
+pkg syscall (linux-386-cgo), type SysProcAttr struct, AmbientCaps []uintptr
 pkg syscall (linux-386), type Credential struct, NoSetGroups bool
+pkg syscall (linux-386), type SysProcAttr struct, AmbientCaps []uintptr
 pkg syscall (linux-amd64-cgo), type Credential struct, NoSetGroups bool
+pkg syscall (linux-amd64-cgo), type SysProcAttr struct, AmbientCaps []uintptr
 pkg syscall (linux-amd64), type Credential struct, NoSetGroups bool
+pkg syscall (linux-amd64), type SysProcAttr struct, AmbientCaps []uintptr
 pkg syscall (linux-arm-cgo), type Credential struct, NoSetGroups bool
+pkg syscall (linux-arm-cgo), type SysProcAttr struct, AmbientCaps []uintptr
 pkg syscall (linux-arm), type Credential struct, NoSetGroups bool
+pkg syscall (linux-arm), type SysProcAttr struct, AmbientCaps []uintptr
 pkg syscall (netbsd-386-cgo), type Credential struct, NoSetGroups bool
 pkg syscall (netbsd-386), type Credential struct, NoSetGroups bool
 pkg syscall (netbsd-amd64-cgo), type Credential struct, NoSetGroups bool

doc/go1.9.html

@@ -784,6 +784,13 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
       to set supplementary groups when starting a new process.
     </p>
 
+    <p><!-- CL 43512 -->
+      The new field
+      <a href="/pkg/syscall/#SysProcAttr.AmbientCaps"><code>SysProcAttr.AmbientCaps</code></a>
+      allows setting ambient capabilities on Linux 4.3+ when creating
+      a new process.
+    </p>
+
     <p><!-- CL 37439 -->
       On 64-bit x86 Linux, process creation latency has been optimized with
       use of <code>CLONE_VFORK</code> and <code>CLONE_VM</code>.