doc: add text/template blocks and redefinition to go1.6.txt

Change-Id: Ide82ac98dc7cb1035ceb9d461ed95af899f8f983
Reviewed-on: https://go-review.googlesource.com/15081Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.6.txt

@@ -18,4 +18,5 @@ text/template: ExecError (https://golang.org/cl/13957/)
 text/template: trimming spaces (https://golang.org/cl/14391/)
 text/template: Funcs check names (https://golang.org/cl/14562/)
 text/template: IsTrue (https://golang.org/cl/14562/)
+text/template: blocks and permit redefinition (https://golang.org/cl/14005)
 time: allow one and two-digit days of the month during Parse (https://golang.org/cl/14123/)