cmd/doc: incorrect handling of some embedded interfaces

[dsnet]

Using go1.7rc2

1082184 caused a regression from Go1.6.

Currently I see:

$ 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
}

I expect to see:

$ 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
}