Improve documentation for composeMultiArrayMessage to clarify usage

[hihry]

Description

Addresses #3243

This PR improves the documentation for the composeMultiArrayMessage function in common.cpp within the moveit_servo module.

• Added a Doxygen-compatible comment block to clarify the function’s purpose, parameters, return value, and usage.
• Helps developers understand how to use this function when working with controllers that accept std_msgs::msg::Float64MultiArray messages (rather than trajectory_msgs::msg::JointTrajectory).
• Enhances clarity and aligns with MoveIt 2’s commitment to being an accessible, industry-standard robotics framework.

This change is purely a documentation update and does not affect runtime behavior.

[sea-bass]

The header file already has the docstrings. Please add it to the one there.

[sea-bass]

Thanks! I was just checking this on some downtime since the same issue has come up before. Once an active maintainer approves, you should be all set.

[github-actions]

This PR is stale because it has been open for 45 days with no activity. Please tag a maintainer for help on completing this PR, or close it if you think it has become obsolete.