spec: clarify that each block has its own version of iota

Issue #15550 is clearly an esoteric case but the spec was silent
about it and we had diverging implementations. By making `iota`
and index that is relative to the respective constant declaration,
nested const declarations won't affect outer values of `iota`.

cmd/compile and go/types already follow this semantics.

Fixes #15550.

Change-Id: If138189e3ea4373f8ba50ac6fb1d219b481f8698
Reviewed-on: https://go-review.googlesource.com/71750Reviewed-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 October 17, 2017",
+	"Subtitle": "Version of October 18, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -1866,8 +1866,8 @@ const (
 <p>
 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
 <code>iota</code> represents successive untyped integer <a href="#Constants">
-constants</a>. It is reset to 0 whenever the reserved word <code>const</code>
-appears in the source and increments after each <a href="#ConstSpec">ConstSpec</a>.
+constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
+in that constant declaration, starting at zero.
 It can be used to construct a set of related constants:
 </p>