Document interaction between Type-Adapters and intra-process communication

[ottojo]

Rclcpp seems to have implemented intra-process communication without serializing custom data types to ROS messages when the apropriate type adapters are used: ros2/rclcpp#1664, but this is not documented.

The type adapter is already used in the subscriber and publisher examples.

The demos repo contains an adapter for cv::Mat (header, implementation) which is used in cam2image.cpp

[ottojo]

I found more information in this comment to the PR implementing this feature: ros2/rclcpp#1849 (comment)

[clalancette]

Thanks, appreciated. I put it on the list of things to look at during the Humble documentation/release process.