docs: event.newGuest for new-window in WebContents and webContents in BrowsweWindow's constructor (#23355)

* docs: `newGuest` in `WebContents` and `webContents` in `BrowsweWindow`

According to the example codes in the documentation of `new-window`
event in `WebContents`, `webContents` in `BrowsweWindow` constructor
options and `newGuest` in `event` argument of `new-window` handler are
both existing but documented. This patch is for adding the related
documentations. Also, it provides typescript-definitations for these
two properties.

* Remove the documnent of `webContents` in BrowserWindow constructor option.

Co-authored-by: Sean Lee <sean.l@canva.com>