Update doc for `OpCodes.Constrained` to "support" static abstract interface members.

[gurustron]

The doc explicitly says that this opcode can appear only a callvirt instruction:

The constrained prefix is permitted only on a callvirt instruction.

But according to sharplab the following:

public interface ITestable {
    public abstract static void Test();
}

public class MyTester<T> where T : ITestable {
    public void RunTest() {
        T.Test();
    }
}

Leads to the following IL:

    .method public hidebysig 
        instance void RunTest () cil managed 
    {
        // Method begins at RVA 0x208e
        // Code size 12 (0xc)
        .maxstack 8

        IL_0000: constrained. !T
        IL_0006: call void ITestable::Test()
        IL_000b: ret
    } // end of method MyTester`1::RunTest

Assuming that sharplab is right and this is intended behaviour it seems that update for docs can be needed.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[gurustron]

Also see this:

II.15.2 Static, Instance and Virtual Methods (page 177)

(Clarify first paragraph)

Static methods are methods that are associated with a type, not with its instances. For
static virtual methods, the particular method to call is determined via a type lookup
based on the constrained. IL instruction prefix or on generic type constraints but
the call itself doesn't involve any instance or this pointer.