cmd/doc: handle embedded interfaces properly

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/25365


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