syscall: freeze the package

Add a clause to the doc comment for the package and a
paragraph in the compatibility document explaining the
situation.

LGTM=bradfitz, adg, rsc
R=golang-codereviews, adg, bradfitz, minux, rsc
CC=golang-codereviews
https://golang.org/cl/129820043

doc/go1compat.html

@@ -152,6 +152,20 @@ will be tagged as appropriate to identify versions that are compatible
 with the Go 1 point releases.
 </p>
 
+<h2 id="operating_systems">Operating systems</h2>
+
+<p>
+It is impossible to guarantee long-term compatibility with operating
+system interfaces, which are changed by outside parties.
+The <a href="/pkg/syscall/"><code>syscall</code></a> package
+is therefore outside the purview of the guarantees made here.
+As of Go version 1.4, the <code>syscall</code> package is frozen.
+Any evolution of the system call interface must be supported elsewhere,
+such as in the <code>go.sys</code> subrepository.
+For details and background, see
+<a href="https://golang.org/s/go1.4-syscall">this document</a>.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <p>

src/pkg/syscall/syscall.go

@@ -17,6 +17,13 @@
 // These calls return err == nil to indicate success; otherwise
 // err is an operating system error describing the failure.
 // On most systems, that error has type syscall.Errno.
+//
+// NOTE: This package is locked down. Code outside the standard
+// Go repository should be migrated to use the corresponding
+// package in the go.sys subrepository. That is also where updates
+// required by new systems or versions should be applied.
+// See https://golang.org/s/go1.4-syscall for more information.
+//
 package syscall
 
 // StringByteSlice is deprecated. Use ByteSliceFromString instead.