ScrolledWindow: Streamline/clarify the intro docs

ScrolledWindow: Streamline/clarify the intro docs

 • intro: Clarify that external policy and/or adjustments can be used.
 • add(): Don’t waffle on about having to add a ViewPort since we handle
          that transparently for the user, so they can add() any widget.
 • Adjustment stuff: most of this was repeating the docs for Scrollbar,
                     so just refer the user to that. Also, mention how
                     policies NEVER and EXTERNAL interact with all this.

https://bugzilla.gnome.org/show_bug.cgi?id=778853