Update irker documentation to mention an irker bug with key-protected channels.

What does this MR do?

Improve a line in the documentation about the irker integration.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

In the webui there is already a line mentioning this irker bug, but the actual documentation lacked it, whereas it should be there.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • not needed, IMHO
• Documentation created/updated
• API support added
  • not relevant
• Tests added for this feature/bug
  • not relevant
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• Seriously this checklist is so damn long for this stupid a doc update, I can't even figure what to put in it...

What are the relevant issue numbers?

None.