Clarify conformance with respect to additional types, members, attribute targets

[jskeet]

Fixes #870

(I had to have a couple of goes at the "additional targets" part - feel free to nitpick the exact wording.)

[Nigel-Ecma]

LGTM

[KalleOlaviNiemitalo]

Side note. "provided they do not alter the behavior of any strictly conforming program" seems like it could interfere with using static. Something like

using static A;

namespace B {
    using static System.Int32;
    class C {
        static int Main() => MaxValue;
    }
}

class A {
    public const int MaxValue = 42;
}

Now if the implementation extends System.Int32 with a public const int MaxValue member, then Main() will return that rather than A.MaxValue, thus changing the behaviour. So the rule seems to pretty much disallow providing additional static members. Or is this kind of program not strictly conforming?