Changes made:
* Disallow star expression on interfaces as this is not possible.
* Show an embedded "error" in an interface as public similar to
how godoc does it.
* Properly handle selector expressions in both structs and interfaces.
This is possible since a type may refer to something defined in
another package (e.g. io.Reader).

Before:
<<<
$ go doc runtime.Error
type Error interface {

    // RuntimeError is a no-op function but
    // serves to distinguish types that are run time
    // errors from ordinary errors: a type is a
    // run time error if it has a RuntimeError method.
    RuntimeError()
    // Has unexported methods.
}

$ go doc compress/flate Reader
doc: invalid program: unexpected type for embedded field
doc: invalid program: unexpected type for embedded field
type Reader interface {
    io.Reader
    io.ByteReader
}
>
> After:
> <<<
> $ go doc runtime.Error
> type Error interface {
>     error
>
>     // RuntimeError is a no-op function but
>     // serves to distinguish types that are run time
>     // errors from ordinary errors: a type is a
>     // run time error if it has a RuntimeError method.
>     RuntimeError()
> }
>
> $ go doc compress/flate Reader
> type Reader interface {
>     io.Reader
>     io.ByteReader
> }

Fixes #16567

Change-Id: I272dede971eee9f43173966233eb8810e4a8c907
Reviewed-on: https://go-review.googlesource.com/25365Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>