Support generating JsonSchema for Polymorphic fields.

This commit introduces MergedJsonSchema and MergedJsonSchemaProperty that can be used to merge properties of multiple objects into one as long as the additions do not conflict with another (eg. due to usage of different types).
To resolve previously mentioned errors it is required to provide a ConflictResolutionFunction.

Closes #3870
Original pull request: #3986.

src/main/asciidoc/reference/mongo-json-schema.adoc

@@ -190,6 +190,103 @@ unless there is more specific information available via the `@MongoId` annotatio
 
 |===
 
+The above example demonstrated how to derive the schema from a very precise typed source.
+Using polymorphic elements within the domain model can lead to inaccurate schema representation for `Object` and generic `<T>` types, which are likely to represented as `{ type : 'object' }` without further specification.
+`MongoJsonSchemaCreator.specify(...)` allows to define additional types that should be considered when rendering the schema.
+
+.Specify additional types for properties
+====
+[source,java]
+----
+public class Root {
+	Object value;
+}
+
+public class A {
+	String aValue;
+}
+
+public class B {
+	String bValue;
+}
+MongoJsonSchemaCreator.create()
+    .specify("value").types(A.class, B.class) <1>
+----
+
+[source,json]
+----
+{
+    'type' : 'object',
+    'properties' : {
+        'value' : {
+            'type' : 'object',
+            'properties' : { <1>
+                'aValue' : { 'type' : 'string' },
+                'bValue' : { 'type' : 'string' }
+            }
+        }
+    }
+}
+----
+<1> Properties of the given types are combined into one element.
+====
+
+MongoDBs schema free approach allows to store documents of different structure in one collection.
+Those may be modeled having a common base class.
+Regardless of the chosen approach `MongoJsonSchemaCreator.combine(...)` is can help circumvent the need of combining multiple schema into one.
+
+.Combining multiple Schemas
+====
+[source,java]
+----
+public abstract class Root {
+	String rootValue;
+}
+
+public class A extends Root {
+	String aValue;
+}
+
+public class B extends Root {
+	String bValue;
+}
+
+MongoJsonSchemaCreator.combined(A.class, B.class) <1>
+----
+
+[source,json]
+----
+{
+    'type' : 'object',
+       'properties' : { <1>
+           'rootValue' : { 'type' : 'string' },
+           'aValue' : { 'type' : 'string' },
+           'bValue' : { 'type' : 'string' }
+       }
+    }
+}
+----
+<1> Properties (and their inherited ones) of the given types are combined into one schema.
+====
+
+[NOTE]
+====
+Equally named properties need to refer to the same json schema in order to be combined.
+The following example shows a definition that cannot be combined automatically because of a data type mismatch.
+In this case a `ConflictResolutionFunction` has to be provided to `MongoJsonSchemaCreator`.
+
+[source,java]
+----
+public class A extends Root {
+	String value;
+}
+
+public class B extends Root {
+	Integer value;
+}
+----
+====
+
 [[mongo.jsonSchema.query]]
 ==== Query a collection for matching JSON Schema