📝 docs(custom.mdx): add description of the 'repr_value' attribute in the CustomComponent class

📝 docs(custom-component.mdx): update example code and description of the 'repr_value' attribute in the DocumentProcessor class

docs/docs/components/custom.mdx

@@ -79,6 +79,12 @@ The CustomComponent class serves as the foundation for creating custom component
   | _`get_flow`_   | Returns a Flow object. Parameters are _`flow_name`_ or _`flow_id`_. |
   | _`load_flow`_  | Loads a flow from a given _`id`_.                                   |
 
+- Useful attributes:
+
+  | Attribute Name | Description                                                                   |
+  | -------------- | ----------------------------------------------------------------------------- |
+  | _`repr_value`_ | Displays the value it receives in the _`build`_ method. Useful for debugging. |
+
   <Admonition type="info" label="Tip">
 
   Check out the [FlowRunner](../examples/flow-runner) example to understand how to call a flow from a custom component.