Add documentation to distinguish agnocast-only APIs from rclcpp-compatible ones

[Koichi98]

Agnocast's internal data structures (e.g., NodeBase, other node interfaces, executors) contain both rclcpp-compatible methods and agnocast-only extensions (e.g., NodeBase::set_on_callback_group_created).

Currently there is no way to tell them apart at a glance. Add documentation comments (e.g., @note agnocast-only) to clearly mark agnocast-specific APIs, making it easier to maintain API compatibility with rclcpp.

[Koichi98]

#1100