doc: describe change to eliminate method expression wrappers from stack traces

Change-Id: I824b42a1c1fdcee8712681ffc6316470761be065
Reviewed-on: https://go-review.googlesource.com/c/159858Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -232,6 +232,23 @@ for {
 </pre>
 </p>
 
+<p><!-- CL 153477 -->
+  Wrappers generated by the compiler to implement method expressions
+  are no longer reported
+  by <a href="/pkg/runtime/#CallersFrames"><code>runtime.CallersFrames</code></a>
+  and <a href="/pkg/runtime/#Stack"><code>runtime.Stack</code></a>. They
+  are also not printed in panic stack traces.
+
+  This change aligns the <code>gc</code> toolchain to match
+  the <code>gccgo</code> toolchain, which already elided such wrappers
+  from stack traces.
+
+  Clients of these APIs might need to adjust for the missing
+  frames. For code that must interoperate between 1.11 and 1.12
+  releases, you can replace the method expression <code>x.M</code>
+  with the function literal <code>func (...) { x.M(...) } </code>.
+</p>
+
 <p><!-- CL 144340 -->
   The compiler now accepts a <code>-lang</code> flag to set the Go language
   version to use. For example, <code>-lang=go1.8</code> causes the compiler to