text/template: Document that range can be used on chan types.

Fixes #4640.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7082044

src/pkg/text/template/doc.go

@@ -63,16 +63,16 @@ data, defined in detail below.
 		otherwise, T1 is executed.  Dot is unaffected.
 
 	{{range pipeline}} T1 {{end}}
-		The value of the pipeline must be an array, slice, or map. If
-		the value of the pipeline has length zero, nothing is output;
+		The value of the pipeline must be an array, slice, map, or channel.
+		If the value of the pipeline has length zero, nothing is output;
 		otherwise, dot is set to the successive elements of the array,
 		slice, or map and T1 is executed. If the value is a map and the
 		keys are of basic type with a defined order ("comparable"), the
 		elements will be visited in sorted key order.
 
 	{{range pipeline}} T1 {{else}} T0 {{end}}
-		The value of the pipeline must be an array, slice, or map. If
-		the value of the pipeline has length zero, dot is unaffected and
+		The value of the pipeline must be an array, slice, map, or channel.
+		If the value of the pipeline has length zero, dot is unaffected and
 		T0 is executed; otherwise, dot is set to the successive elements
 		of the array, slice, or map and T1 is executed.