Document differences between rclcpp_components_register_node and rclcpp_components_register_nodes (#5984)

Signed-off-by: Maurice <[email protected]>
Co-authored-by: Alejandro Hernández Cordero <[email protected]>

Author: mini-1235

source/Concepts/Intermediate/About-Composition.rst

@@ -84,6 +84,33 @@ For an example, :doc:`check out this tutorial <../../Tutorials/Intermediate/Writ
 
    In order for the component_container to be able to find desired components, it must be executed or launched from a shell that has sourced the corresponding workspace.
 
+CMake Registration Macros
+-------------------------
+
+ROS 2 provides two CMake macros for registering components, each serving a different purpose:
+
+``rclcpp_components_register_node``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+This macro registers a component and generates a standalone executable.
+Use this when you want both composability and the ability to run the node as a standalone process.
+
+.. code-block:: cmake
+
+   add_library(talker_component SHARED src/talker_component.cpp)
+   rclcpp_components_register_node(talker_component
+     PLUGIN "composition::Talker"
+     EXECUTABLE talker)
+
+``rclcpp_components_register_nodes``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+This macro registers one or more components for runtime composition **without** creating standalone executables.
+Use this when you want pure component libraries that will be loaded into component containers at runtime.
+
+.. code-block:: cmake
+
+   add_library(talker_component SHARED src/talker_component.cpp)
+   rclcpp_components_register_nodes(talker_component "composition::Talker")
+
 Using Components
 ----------------