Clarify what retainContextWhenHidden does to webview scripts when switching focus

[octref]

#48453

Currently, with retainContextWhenHidden: true, and I have an editor with a webview next to each other, this happens:

• Cat keeps counting to 40
• When I switch focus to editor, cat stops counting
• When I switch back to the webview, cat starts counting from 40

This says the script code stops execution when I switch away from webview. However, a common understanding of "background-script" is that it keeps running in the background. I believe it needs to be made clear:

• What happens to a running script when you switch focus away
• What get restored and how they get restored. Do you take a snapshot of the DOM and restore the DOM and rerun the script? Do you keep the script execution context and restore it? Those remain a bit vague.