docs(core-concepts): add info about using detached: true in store subscriptions inside plugins

[F0rsaken]

Added note in docs about using detached: true with $subscribe and $onAction inside plugins in Core conepts -> plugins to help people avoid problems mentioned in this issue #991 about losing subscriptions after a component is destroyed.

[netlify]

✅ Deploy Preview for pinia-official ready!

Name	Link
🔨 Latest commit	286a1e7
🔍 Latest deploy log	https://app.netlify.com/sites/pinia-official/deploys/636906ae76f8260009fbf4c6
😎 Deploy Preview	https://deploy-preview-1781--pinia-official.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[posva]

Thanks for this. After some inspection I realized there is no problem, I even wrote a test for it in the subscriptions spec... I'm sorry I asked you to create a PR for this. The plugins in pinia are ran within the store's effect scope, which means they are disconnected from a regular $subscribe() in a component's setup...

I refactored the code at 57405de but it seems like it works without the change. If you found a case that doesn't work, a bug report with a reproduction would help.