Add clarity for Openhands-resolver guide

[malhotra5]

End-user friendly description of the problem this fixes or functionality that this introduces

• [ ] Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions I've noticed two issues

1. There is some confusion regarding when to use the fix-me label versus @openhands-agent macro
2. Some folks outside the Openhands Slack have been unaware that PR feedback can be addresses using openhands-resolver

This PR adds additional usage tips to the Openhands docs

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Link of any specific issues this addresses

5035

[neubig]

(sorry, that was actually me :))