clarify requirements on the target instance; this commit should be sq…

…uashed before merging

spec/src/main/asciidoc/core/invokers.asciidoc

@@ -47,6 +47,7 @@ public interface Invoker<T, R> {
 ----
 
 Calling `invoke()` invokes the target method on given `instance` of the target bean, passing given `arguments`, and propagates back the return value or thrown exception.
+The `instance` may be a non-contextual object.
 
 Invoker implementations must be thread-safe.
 A single invoker instance may be used to perform multiple, possibly concurrent, invocations of the target method, possibly on different instances of the target bean, possibly with different arguments.
@@ -57,8 +58,16 @@ Whenever a direct invocation of a method on an object is a business method invoc
 
 If the target method is `static`, the `instance` is ignored; by convention, it should be `null`.
 If the target method is not `static` and `instance` is `null`, a `RuntimeException` is thrown.
-If the target method is not `static` and the class of the `instance` is not a subclass of the bean class of the target bean, a `RuntimeException` is thrown.
+If the target method is not `static` and the class of the `instance` makes it impossible to invoke the target method, a `RuntimeException` is thrown.
 
+[NOTE]
+====
+Usually, the class of the `instance` should be the bean class of the target bean (or a subclass), in which case, it is possible to invoke all target methods.
+This is true when `instance` is a contextual reference for the target bean obtained using the bean class as the required type.
+
+It is however permissible for the `instance` to be a reference to a type that cannot be cast to the bean class (see <<typecasting_between_bean_types>>).
+In this case, invoking some target methods may be impossible.
+====
 
 Correspondence between given `arguments` and declared parameters of the target method is positional: the Nth element of the `arguments` array is passed as the Nth argument to the target method.
 If the target method is a variable arity method, the last element of the `arguments` array corresponds to the variable arity parameter (and therefore must be an array).