[vm/ffi] Struct documentation issue: Array may be part of the Struct

[sgrekhov]

Struct documentation reads

All field declarations in a Struct subclass declaration must either have type int or double and be annotated with a NativeType representing the native type, or must be of type Pointer.

But 'Struct' may also contain 'Array' type fields or subtypes of Struct or Union

Please update the documentation accordingly.

Also please see #44935

[sgrekhov]

cc @dcharkes

[sgrekhov]

Btw what about bool it Struct? The following program works well

class C extends Struct {
  @Bool()
  external bool b;
}

main() {
  Pointer<C> p = calloc<C>();
  try {
    p.ref.b = false;
    print(p.ref.b);
  } finally {
    calloc.free(p);
  }
}

Dart SDK version: 2.16.0-edge.3d1f6a48b79e2cc5a4ee9d2ea3a7a385a3b33eaa (be) (Thu Nov 18 00:58:40 2021 +0000) on "windows_x64"