Document sharing one DriftIsolate via IsolateNameServer #2095

[AzazelSensei]

People hitting this usually spawn a DriftIsolate in the UI and again in a background service. Those are two databases, so watch() in the foreground never sees the background writes.

This adds that warning to the independent-isolates section, plus the lookup-or-spawn snippet using IsolateNameServer that you already confirmed on the issue. Flutter apps in the same engine can keep using shareAcrossIsolates: true instead.

Fixes #2095

[simolus3]

Thanks for improving documentation on this!

Reading the snippet though, I wonder if we should instead add a link to drift_flutter sources as an inspiration instead of adding a standalone snippet. IsolateNameServer is a brittle API, and one needs to defend against cases like a hot restart where some isolates are restarted but the dead port is still registered in the isolate server (drift_flutter uses timeouts and pings for that).

But more importantly, IsolateNameServer only exists for Flutter and drift_flutter already does the right thing. So I suppose what the docs should suggest is a stronger focus on using drift_flutter instead of trying to wire up isolates manually.