cmd/doc: allow -all to apply to individual items

It really only matters for types, and the code already worked but was
blocked by a usage check.

Fixes #25595

Change-Id: I823f313b682b37616ea555aee079e2fe39f914c2
Reviewed-on: https://go-review.googlesource.com/c/144357Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/doc/doc_test.go

@@ -481,6 +481,26 @@ var tests = []test{
 			`unexportedTypedConstant`,       // No unexported constant.
 		},
 	},
+	// Type -all.
+	{
+		"type",
+		[]string{"-all", p, `ExportedType`},
+		[]string{
+			`type ExportedType struct {`,                        // Type definition as source.
+			`Comment about exported type`,                       // Include comment afterwards.
+			`const ConstGroup4 ExportedType = ExportedType\{\}`, // Related constants.
+			`ExportedTypedConstant ExportedType = iota`,
+			`Constants tied to ExportedType`,
+			`func ExportedTypeConstructor\(\) \*ExportedType`,
+			`Comment about constructor for exported type.`,
+			`func ReturnExported\(\) ExportedType`,
+			`func \(ExportedType\) ExportedMethod\(a int\) bool`,
+			`Comment about exported method.`,
+		},
+		[]string{
+			`unexportedType`,
+		},
+	},
 	// Type T1 dump (alias).
 	{
 		"type T1",

src/cmd/doc/main.go

@@ -133,10 +133,7 @@ func do(writer io.Writer, flagSet *flag.FlagSet, args []string) (err error) {
 		}
 
 		// We have a package.
-		if showAll {
-			if symbol != "" {
-				return fmt.Errorf("-all valid only for package, not symbol: %s", symbol)
-			}
+		if showAll && symbol == "" {
 			pkg.allDoc()
 			return
 		}