Improve callRealMethod() documentation (spockframework#1005)

* Improve callRealMethod() documentation

Closes spockframework#830.

* A la carte mock needs an IDefaultResponse instance for 'defaultResponse', not a type

See also https://stackoverflow.com/a/57305121/1082681

docs/interaction_based_testing.adoc

@@ -805,6 +805,11 @@ Note that we don't have to pass the `message` argument along; this is taken care
 returns the real invocation's result, but in this example we opted to return our own result instead.
 If we had wanted to pass a different message to the real method, we could have used `callRealMethodWithArgs("changed message")`.
 
+Please note that while semantically both `callRealMethod()` and `callRealMethodWithArgs(...)` only make sense with spies,
+technically you can also call these methods on mock or stub objects, kind of turning them into (pseudo) spy objects
+"through the backdoor". The only precondition is that the mocked/stubbed object actually has a real method implementation,
+i.e. for interface mocks there must be a default method, for class mocks there must be a (non-abstract) original method.
+
 [[PartialMocks]]
 === Partial Mocks
 
@@ -963,7 +968,7 @@ can be passed as named arguments to the `Mock()` method. For example:
 
 [source,groovy]
 ----
-def person = Mock(name: "Fred", type: Person, defaultResponse: ZeroOrNullResponse, verified: false)
+def person = Mock(name: "Fred", type: Person, defaultResponse: ZeroOrNullResponse.INSTANCE, verified: false)
 ----
 
 Here, we create a mock whose default return values match those of a `Mock()`, but whose invocations aren't

spock-core/src/main/java/org/spockframework/mock/IMockInvocation.java

@@ -41,8 +41,9 @@ public interface IMockInvocation {
 
   /**
    * Delegates this method invocation to the real object underlying this mock object,
-   * including any method arguments. If this mock object has no underlying real object, a
-   * {@link CannotInvokeRealMethodException} is thrown.
+   * including any method arguments.
+   * If this mock object has no underlying real (non-interface) object or interface
+   * default method implementation, a {@link CannotInvokeRealMethodException} is thrown.
    *
    * @return the return value of the method to which this invocation was delegated
    */
@@ -51,8 +52,8 @@ public interface IMockInvocation {
   /**
    * Delegates this method invocation to the real object underlying this mock object,
    * replacing the original method arguments with the specified arguments.
-   * If this mock object has no underlying real object, a
-   * {@link CannotInvokeRealMethodException} is thrown.
+   * If this mock object has no underlying real (non-interface) object or interface
+   * default method implementation, a {@link CannotInvokeRealMethodException} is thrown.
    *
    * @return the return value of the method to which this invocation was delegated
    */