encoding/asn1: document "utc" and "generalized" tags

Also reformat tables.

Fixes #19889

Change-Id: I05083d2bab8bca46c4e22a415eb9b73513df6994
Reviewed-on: https://go-review.googlesource.com/40071Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/asn1/marshal.go

@@ -643,10 +643,12 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 // In addition to the struct tags recognised by Unmarshal, the following can be
 // used:
 //
-//	ia5:		causes strings to be marshaled as ASN.1, IA5 strings
+//	ia5:         causes strings to be marshaled as ASN.1, IA5String values
 //	omitempty:   causes empty slices to be skipped
-//	printable:	causes strings to be marshaled as ASN.1, PrintableString strings.
-//	utf8:		causes strings to be marshaled as ASN.1, UTF8 strings
+//	printable:   causes strings to be marshaled as ASN.1, PrintableString values
+//	utf8:        causes strings to be marshaled as ASN.1, UTF8String values
+//	utc:         causes time.Time to be marshaled as ASN.1, UTCTime values
+//	generalized: causes time.Time to be marshaled as ASN.1, GeneralizedTime values
 func Marshal(val interface{}) ([]byte, error) {
 	e, err := makeField(reflect.ValueOf(val), fieldParameters{})
 	if err != nil {