Clarify Plex watchlist rule behavior and cloud lookup limitations

[Copilot]

The docs did not explain that Plex watchlist rules operate on all users returned by the Plex server account list or that these lookups depend on Plex cloud services. This left gaps around private watchlists, removed friends still being scanned, and why Test Media can succeed while full runs time out.

• Rules guidance

  • Added a dedicated Plex watchlist note to Rules.mdx covering:
    • Watchlisted by and Is Watchlisted scanning all Plex users returned by the server account list
    • lack of a per-user exclude mechanism
    • dependency on plex.tv / community.plex.tv, not just the local Plex server
    • Watchlisted by being experimental and more fragile than local metadata rules

• Troubleshooting expectations

  • Documented expected warnings for private watchlists
  • Called out that local Plex IP changes are usually irrelevant for this code path
  • Clarified that repeated cloud lookups during full rule execution can fail even when Test Media succeeds

• Glossary updates

  • Expanded the Watchlisted by (username) and Is Watchlisted glossary entries so the rule behavior and cloud dependency are visible where users look up individual rule definitions

:::warning Plex Watchlist Rules
`Watchlisted by` and `Is Watchlisted` do not only check the current Plex user.
Maintainerr scans every user still returned by the Plex server account list
and then resolves the Plex cloud identities needed for watchlist lookups.
:::