docs(worker): comment purpose of UI refresh task

[ThomasFrans]

The purpose of the manual ui refresh task isn't immediately clear. It was explained in the commit message of the commit that introduced it.

Describe your changes

Add a comment that explains the purpose of the refresh_ui task in the worker thread.

Checklist before requesting a review

• Documentation was updated (i.e. due to changes in keybindings, commands, etc.)
• Changelog was updated with relevant user-facing changes (eg. not dependency updates,
  not performance improvements, etc.)

[ThomasFrans]

Stupidly small PR but I had to 'Git blame' to find what this was used for. Thought I'd add a comment in case others read this and didn't understand either. I assumed the player events were used to update the status bar.