cmd/doc: perform type grouping for constants and variables

In golang.org/cl/22354, we added functionality to group functions under the
type that they construct to. In this CL, we extend the same concept to
constants and variables. This makes the doc tool more consistent with what
the godoc website does.

$ go doc reflect | egrep "ChanDir|Kind|SelectDir"
<<<
// Before:
const RecvDir ChanDir = 1 << iota ...
const Invalid Kind = iota ...
type ChanDir int
type Kind uint
type SelectDir int
    func ChanOf(dir ChanDir, t Type) Type

// After:
type ChanDir int
    const RecvDir ChanDir = 1 << iota ...
type Kind uint
    const Invalid Kind = iota ...
type SelectDir int
    const SelectSend SelectDir ...
    func ChanOf(dir ChanDir, t Type) Type
>>>

Furthermore, a fix was made to ensure that the type was printed in constant
blocks when the iota was applied on an unexported field.

$ go doc reflect SelectSend
<<<
// Before:
const (
	SelectSend    // case Chan <- Send
	SelectRecv    // case <-Chan:
	SelectDefault // default
)

// After:
const (
	SelectSend    SelectDir // case Chan <- Send
	SelectRecv              // case <-Chan:
	SelectDefault           // default
)
>>>

Fixes #16569

Change-Id: I26124c3d19e50caf9742bb936803a665e0fa6512
Reviewed-on: https://go-review.googlesource.com/25419


Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>