chore: add comment to the PLUGIN_DIFY_INNER_API_KEY key

[Nov1c444]

Summary

If PLUGIN_DIFY_INNER_API_KEY in docker/.env is changed, the DIFY_INNER_API_KEY in the plugin_daemon service must be synchronized accordingly, otherwise the Agent node will fail.

Resolves #15118
Resolves #14850

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods