doc/effective_go: clarify backward function reference

Fixes #14656.

Change-Id: I37a9aa51705ae18bd034f2cc6dbf06a55f969197
Reviewed-on: https://go-review.googlesource.com/23202Reviewed-by: Rob Pike <r@golang.org>

doc/effective_go.html

@@ -2014,7 +2014,7 @@ then make the receiver for the method a value of that type.
 type ByteSlice []byte
 
 func (slice ByteSlice) Append(data []byte) []byte {
-    // Body exactly the same as above
+    // Body exactly the same as the Append function defined above.
 }
 </pre>
 <p>