spec: clarify meaning of alias decls in sequence of "enum" const declaration

```
package main

import "math"
import "fmt"

const (
	a = iota
	b
	c => math.Pi
	d
)

func main() {
	fmt.Println(a, b, c, d)
}
```
prints
```
0 1 3.141592653589793 2
```
It probably shouldn't be allowed in the first place. Certainly the output is wrong (iota should be incremented with each semicolon).