Clarify WebviewPanelSerializer doc

[octref]

#50696

interface WebviewPanelSerializer {
  /**
   * Restore a webview panel from its serialized `state`.
   *
   * Called when a serialized webview first becomes visible.
   *
   * @param webviewPanel Webview panel to restore. The serializer should take ownership of this panel.
   * @param state Persisted state. This state comes from the value set inside the webview by `acquireVsCodeApi().setState`.
   *
   * @return Thenable indicating that the webview has been fully restored.
   */
  deserializeWebviewPanel(webviewPanel: WebviewPanel, state: any): Thenable<void>
}

Webview panel to restore.

Well, maybe being too pedantic but the item being stored is the panel's webview, not the panel.

The serializer should take ownership of this panel.

"The serializer should take ownership of this panel" is a bit vague.

Maybe something like "The WebviewPanel whose webview will be restored by the serializer"?

Called when a serialized webview first becomes visible.

I suggest adding some information. It's unclear when this is called. From my testing closing -> opening new WebviewPanel won't get it called, but it'll get called for VS Code reload & restart.

[mjbvz]

Added notes with fae2d73 and will be updating webview example to use persistence APIs