docs: clarify authorisation window when policies change (#50695)

## Solution

Clarifying the authorisation window for RLS/access checks on channels in
Realtime.

<!--
## Preview links

If relevant, include links to changed pages for easy review access.

Copy the preview base URL from the Vercel bot comment on this PR. Use
the following table as an example template.

| Site | Live | Preview | Search for |
| -------------- |
-------------------------------------------------------------------------
|
------------------------------------------------------------------------------------------------------------
| ----------------------------- |
| WWW | [/blog/your-post](https://supabase.com/blog/your-post) |
[/blog/your-post](https://zone-www-dot-com-git-branch-name-supabase.vercel.app/blog/your-post)
| unique phrase from the change |
| Docs |
[/docs/guides/your-page](https://supabase.com/docs/guides/your-page) |
[/docs/guides/your-page](https://docs-git-branch-name-supabase.vercel.app/docs/guides/your-page)
| unique phrase from the change |
| Studio | [/dashboard](https://supabase.com/dashboard) |
[/dashboard](https://studio-git-branch-name-supabase.vercel.app/dashboard)
| unique phrase from the change |
| Design system | [/design-system](https://supabase.com/design-system) |
[/design-system](https://design-system-git-branch-name-supabase.vercel.app/design-system)
| unique phrase from the change |
| UI library | [/library](https://supabase.com/library) |
[/library](https://ui-library-git-branch-name-supabase.vercel.app/library)
| unique phrase from the change |
| Knowledge base |
[/kb/guides/your-page](https://supabase.com/kb/guides/your-page) |
[/kb/guides/your-page](https://kb-git-branch-name-supabase.vercel.app/kb/guides/your-page)
| unique phrase from the change |
-->

<!-- ## Additional context

Optionally add any other context or screenshots.

-->

## Review instructions

Provide a clear numbered procedure that the PR reviewer can walk
through.

1. For example, `Open the live and preview links side-by-side.`
2. For example, `See the issue is fixed.`


## Checklist

Check all before review:

- [x] I have read
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
- [ ] If I wrote a new docs topic or edited an existing topic, I used
the `/write-the-docs` or `/edit-the-docs` skill, which references
[WORD_LIST](https://github.com/supabase/supabase/blob/master/apps/docs/WORD_LIST.md)
and the docs
[CONTRIBUTING](https://github.com/supabase/supabase/blob/master/apps/docs/CONTRIBUTING.md)
guide


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Clarified that Realtime authorization policy changes may not affect
already-connected clients until their JWT expires or a new JWT is
provided.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/docs/content/guides/realtime/authorization.mdx

@@ -402,6 +402,8 @@ Realtime updates the access policy cache for a client based on your RLS policies
 - A client connects to Realtime and subscribes to a Channel
 - A new JWT is sent to Realtime from a client via the [`access_token` message](/docs/guides/realtime/protocol#access-token)
 
+This means that if you revoke a user's access (for example, by removing a row an RLS policy uses to grant it) while they're still connected, they'll keep receiving messages until their JWT expires or a new one is sent.
+
 If a new JWT is never received on the Channel, the client will be disconnected when the JWT expires.
 
 Make sure to keep the JWT expiration window short.