docs: Improve self-heal and auto-sync behaviors explanation for multi-source apps (#20809)

[andrii-korotkov-verkada]

Closes #20809

In single-source apps self-heal disabled guarantees that with no source changes the live changes won't get overridden. That's no longer the case for multi-source apps, since one of the sources changing can trigger a sync of everything. Add a note explaining this behavior.

Checklist:

• [x] 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.
• [x] The title of the PR states what changed and the related issues number (used for the release note).
• [x] The title of the PR conforms to the Toolchain Guide
• [x] 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.
• [x] Does this PR require documentation updates?
• [x] I've updated documentation as required by this PR.
• [x] 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.
• [x] My build is green (troubleshooting builds).
• [ ] My new feature complies with the feature status guidelines.
• [x] I have added a brief description of why this PR is necessary and/or what this PR solves.
• [x] 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).

[bunnyshell[bot]]

:x: Preview Environment deleted from Bunnyshell

Available commands (reply to this comment):

• :rocket: /bns:deploy to deploy the environment