Clarify GitHub integration docs regarding @openhands mentions in pull requests

[neubig]

This PR clarifies the GitHub integration documentation for OpenHands Cloud to note that writing @openhands on a pull request will only work if the pull request is both to and from a repository that you have added through the interface.

The change adds an important note in the "Working with Pull Requests" section of the GitHub installation documentation to make this limitation clear to users.

This addresses the issue where users might be confused when the @openhands mention doesn't work on pull requests from forks or external repositories.

@neubig can click here to continue refining the PR

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:5a4f00f-nikolaik   --name openhands-app-5a4f00f   docker.all-hands.dev/all-hands-ai/openhands:5a4f00f