cmd/doc: group constructors with type in package presentation

cmd/doc: group constructors with type in package presentation

Fixes #14004.

$ go doc encoding.gob
Before:
func Register(value interface{})
func RegisterName(name string, value interface{})
func NewDecoder(r io.Reader) *Decoder
func NewEncoder(w io.Writer) *Encoder
type CommonType struct { ... }
type Decoder struct { ... }
type Encoder struct { ... }
type GobDecoder interface { ... }
type GobEncoder interface { ... }

After:
func Register(value interface{})
func RegisterName(name string, value interface{})
type CommonType struct { ... }
type Decoder struct { ... }
    func NewDecoder(r io.Reader) *Decoder
type Encoder struct { ... }
    func NewEncoder(w io.Writer) *Encoder
type GobDecoder interface { ... }
type GobEncoder interface { ... }

Change-Id: I021db25bce4a16b3dfa22ab323ca1f4e68d50111
Reviewed-on: https://go-review.googlesource.com/22354
Reviewed-by: Robert Griesemer <gri@golang.org>