Clarify duration/end_date_time usage

[FletcherAU]

Proposed change

Add a note to duration that it's mutually exclusive with end_date_time. (the latter of which already had this note)

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase:
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes Incorrect details for service calendar.list_events #29787

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[c0ffeeca7]

@FletcherAU, you marked this PR as draft a while ago. Is it ready for review? Or are you still working on it?

[FletcherAU]

Ideally this change would include a clarification in the Optional column but I'm not sure whether that is in keeping with the standard set by other pages. I was holding off until there was clarification in either #29787 or #dev-documentation :)

[c0ffeeca7]

Thank you, @FletcherAU 👍