html/template: adjust ambiguous URL context text

Before: ... appears in an ambiguous URL context.
After:  ... appears in an ambiguous context within a URL.

It's a minor point, but it's confused multiple people.
Try to make clearer that the ambiguity is "where exactly inside the URL?"

Fixes #17319.

Change-Id: Id834868d1275578036c1b00c2bdfcd733d9d2b7b
Reviewed-on: https://go-review.googlesource.com/31465
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/html/template/error.go

@@ -44,7 +44,7 @@ const (
 	// OK indicates the lack of an error.
 	OK ErrorCode = iota
 
-	// ErrAmbigContext: "... appears in an ambiguous URL context"
+	// ErrAmbigContext: "... appears in an ambiguous context within a URL"
 	// Example:
 	//   <a href="
 	//      {{if .C}}

src/html/template/escape.go

@@ -161,7 +161,7 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		case urlPartUnknown:
 			return context{
 				state: stateError,
-				err:   errorf(ErrAmbigContext, n, n.Line, "%s appears in an ambiguous URL context", n),
+				err:   errorf(ErrAmbigContext, n, n.Line, "%s appears in an ambiguous context within a URL", n),
 			}
 		default:
 			panic(c.urlPart.String())

src/html/template/escape_test.go

@@ -903,7 +903,7 @@ func TestErrors(t *testing.T) {
 		},
 		{
 			`<a href="{{if .F}}/foo?a={{else}}/bar/{{end}}{{.H}}">`,
-			"z:1:47: {{.H}} appears in an ambiguous URL context",
+			"z:1:47: {{.H}} appears in an ambiguous context within a URL",
 		},
 		{
 			`<a onclick="alert('Hello \`,