Add warning to issue description modal

What does this MR do and why?

Add warning to issue description modal

Note: Other UI changes to this modal are in UI updates- summarize comments and description (!122103 - merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• AI set up
• Feature flags:
  • :summarize_notes
  • :summarize_comments
  • :generate_description_ai
• Go to issue
• Click edit > Autocomplete

#411677 (closed)