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/asn1.go

@@ -975,12 +975,12 @@ func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
 //
 // The following tags on struct fields have special meaning to Unmarshal:
 //
-//	application	specifies that a APPLICATION tag is used
-//	default:x	sets the default value for optional integer fields (only used if optional is also present)
-//	explicit	specifies that an additional, explicit tag wraps the implicit one
-//	optional	marks the field as ASN.1 OPTIONAL
-//	set		causes a SET, rather than a SEQUENCE type to be expected
-//	tag:x		specifies the ASN.1 tag number; implies ASN.1 CONTEXT SPECIFIC
+//	application specifies that a APPLICATION tag is used
+//	default:x   sets the default value for optional integer fields (only used if optional is also present)
+//	explicit    specifies that an additional, explicit tag wraps the implicit one
+//	optional    marks the field as ASN.1 OPTIONAL
+//	set         causes a SET, rather than a SEQUENCE type to be expected
+//	tag:x       specifies the ASN.1 tag number; implies ASN.1 CONTEXT SPECIFIC
 //
 // If the type of the first field of a structure is RawContent then the raw
 // ASN1 contents of the struct will be stored in it.

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
-//	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
+//	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 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 {