more info about IW specifics

Author: amunger

src/vs/workbench/contrib/interactive/browser/docs/interactive.md

@@ -30,6 +30,10 @@ When the `SimpleNotebookEditorModel` is requested to `load`, it will ask the `Wo
 
 ![editor registration](interactive.model.resolution.drawio.svg)
 
+The `FileSystem` provider that is registered for `vscode-interactive` schema will always return an empty buffer for any read, and will drop all write requests as nothing is stored on disk for Interactive Window resources. The `notebookSerializer` that is registered for the `interactive` viewtype knows to return an empty notebook data model when it deserializes an empty buffer when the model is being resolved.
+
+Restoring the interactive window happens through the `EditorModelCache`, where the full notebook data is stored, and can be used to repopulate the `EditorInput` without needing to go through the editor model resolution flow, effectively skipping any filesystem reads.
+
 ## UI/EH editor/document syncing
 
 `EditorInput` is responsible for resolving models for the given resources but in Interactive Window it's much simpler as we are not resolving models ourselves but delegating to Notebook and TextEditor. `InteractiveEditorInput` does the coordination job.