docs: Update documentation for orphaned resource count metric

I have updated the documentation as a follow-up to PR #20521

Checklist:

[ ] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
[ ] The title of the PR states what changed and the related issues number (used for the release note).
[ ] The title of the PR conforms to the Toolchain Guide
[ ] I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
[ ] I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
[ ] Does this PR require documentation updates?
[ ] I've updated documentation as required by this PR.
[ ] I have signed off all my commits as required by DCO
[ ] I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
[ ] My build is green (troubleshooting builds).
[ ] My new feature complies with the feature status guidelines.
[ ] I have added a brief description of why this PR is necessary and/or what this PR solves.
[ ] Optional. My organization is added to USERS.md.
[ ] Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

argoproj / argo-cd