Clean up container resource builder extensions XML docs

[IEvangelist]

Description

• Correct code example embedded in triple slash
• Consistent return defs
• Better formatting for code example too.

Similar to #6753

Checklist

• Is this feature complete?
  • [x] Yes. Ready to ship.
  • [ ] No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • [ ] Yes
  • [x] No
• Did you add public API?
  • [ ] Yes
  • If yes, did you have an API Review for it?
    • [ ] Yes
    • [ ] No
  • Did you add <remarks /> and <code /> elements on your triple slash comments?
    • [ ] Yes
    • [ ] No
  • [x] No
• Does the change make any security assumptions or guarantees?
  • [ ] Yes
  • If yes, have you done a threat model and had a security review?
    • [ ] Yes
    • [ ] No
  • [x] No

• Does the change require an update in our Aspire docs?

  • [ ] Yes
  • Is this introducing a breaking change?
    • [ ] Yes
    • Link to aspire-docs issue (please use this breaking-change template):
    • [ ] No
    • Link to aspire-docs issue (please use this doc-idea template):
  • [x] No

  Microsoft Reviewers: Open in CodeFlow