doc: mention availability of race detector in go1.1 doc

R=r
CC=golang-dev
https://golang.org/cl/8301044

doc/go1.1.html

@@ -291,6 +291,8 @@ The byte-order-mark change is strictly backward-compatible.
 A major addition to the tools is a <em>race detector</em>, a way to find
 bugs in programs caused by problems like concurrent changes to the same variable.
 This new facility is built into the <code>go</code> tool.
+For now, it is only available on Linux, Mac OS X, and Windows systems with
+64-bit x86 processors.
 To enable it, set the <code>-race</code> flag when building or testing your program 
 (for instance, <code>go test -race</code>).
 The race detector is documented in <a href="/doc/articles/race_detector.html">a separate article</a>.