docs(joint_state_broadcaster): clarify /dynamic_joint_states contents (backport #1865)

[mergify]

This PR updates the existing doc/userdoc.rst for the joint_state_broadcaster.

What’s included

• Added a new "Published topics" section:
  • /joint_states: publishes movement-related interfaces (position, velocity, effort).
  • /dynamic_joint_states: publishes all available state interfaces per joint,
    including custom ones (e.g., temperature, voltage, torque sensor readings).
• Added a YAML-style example payload for clarity.
• Added a note about the use_local_topics parameter.

Why

• The previous documentation only stated that both topics are published but did not
  explain their contents in detail.
• This clarification makes the documentation consistent with demos and API expectations
  and answers common questions from new users.

Fixes #1716

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This is an automatic backport of pull request #1865 done by [Mergify](https://mergify.com).