spec: for non-constant map keys, add reference to evaluation order section

The section on map literals mentions constant map keys but doesn't say
what happens for equal non-constant map keys - that is covered in the
section on evaluation order. Added respective link for clarity.

Fixes #19689.

Change-Id: If9a5368ba02e8250d4bb0a1d60d0de26a1f37bbb
Reviewed-on: https://go-review.googlesource.com/38598Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 12, 2017",
+	"Subtitle": "Version of March 24, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -2295,7 +2295,8 @@ The key is interpreted as a field name for struct literals,
 an index for array and slice literals, and a key for map literals.
 For map literals, all elements must have a key. It is an error
 to specify multiple elements with the same field name or
-constant key value.
+constant key value. For non-constant map keys, see the section on
+<a href="#Order_of_evaluation">evaluation order</a>.
 </p>
 
 <p>