You're reading the documentation for an older, but still supported, version of ROS 2. For information on the latest version, please have a look at Iron.
All ROS 2 package’s documentation is hosted alongside its information on the ROS Index.
Searching for packages will yield their information such as released distributions,
README.md files, URLs, and other important metadata.
A list of all packages for each distribution on the index can be found here.
From the right-hand side of a package of interest, the auto-generated API documentation may be found found using the “API Docs” button.
You can find the API Documentation for the Client Libraries in the Humble distribution using the links below:
A raw list of Humble package documentation may be found here.