cmd/doc: print Go syntax when printing struct.field docs

Fixes #20928

Change-Id: I7f7aafb8ff4b5deb50c286a9ae81c34ee85e56a9
Reviewed-on: https://go-review.googlesource.com/47730
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/doc/doc_test.go

@@ -89,7 +89,7 @@ var tests = []test{
 			`VarFive = 5`,                       // From block starting with unexported variable.
 			`type unexportedType`,               // No unexported type.
 			`unexportedTypedConstant`,           // No unexported typed constant.
-			`Field`,                             // No fields.
+			`\bField`,                           // No fields.
 			`Method`,                            // No methods.
 			`someArgument[5-8]`,                 // No truncated arguments.
 			`type T1 T2`,                        // Type alias does not display as type declaration.
@@ -395,14 +395,16 @@ var tests = []test{
 		"field",
 		[]string{p, `ExportedType.ExportedField`},
 		[]string{
+			`type ExportedType struct`,
 			`ExportedField int`,
 			`Comment before exported field.`,
 			`Comment on line with exported field.`,
+			`other fields elided`,
 		},
 		nil,
 	},
 
-	// Field  with -u.
+	// Field with -u.
 	{
 		"method with -u",
 		[]string{"-u", p, `ExportedType.unexportedField`},
@@ -413,6 +415,14 @@ var tests = []test{
 		nil,
 	},
 
+	// Field of struct with only one field.
+	{
+		"single-field struct",
+		[]string{p, `ExportedStructOneField.OnlyField`},
+		[]string{`the only field`},
+		[]string{`other fields elided`},
+	},
+
 	// Case matching off.
 	{
 		"case matching off",