spec: removed TODOs (invisible html comment) in favor of issues

- no "visible" change to spec but for updated date
- retired several outdated TODO items
- filed non-urgent issues 10953, 10954, 10955 for current TODOs

Change-Id: If87ad0fb546c6955a6d4b5801e06e5c7d5695ea2
Reviewed-on: https://go-review.googlesource.com/10382Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 18, 2015",
+	"Subtitle": "Version of May 26, 2015",
 	"Path": "/ref/spec"
 }-->
 
-<!--
-TODO
-[ ] need language about function/method calls and parameter passing rules
-[ ] last paragraph of #Assignments (constant promotion) should be elsewhere
-    and mention assignment to empty interface.
-[ ] need to say something about "scope" of selectors?
-[ ] clarify what a field name is in struct declarations
-    (struct{T} vs struct {T T} vs struct {t T})
-[ ] need explicit language about the result type of operations
-[ ] should probably write something about evaluation order of statements even
-	though obvious
-[ ] in Selectors section, clarify what receiver value is passed in method invocations
--->
-
-
 <h2 id="Introduction">Introduction</h2>
 
 <p>