Document custom scale factor in Multiple resolutions

- Mention how to change each stretch-related property at runtime
  from a script.
- Link to the Multiple Resolutions and Aspect Ratios demo project.
- Remove outdated Shrink section (this is no longer present in 4.0,
  as it was replaced by Scale).

tutorials/rendering/multiple_resolutions.rst

@@ -50,6 +50,11 @@ handle scaling for different sizes and aspect ratios.
 
 Godot provides several useful tools to do this easily.
 
+.. seealso::
+
+    You can see how Godot's support for multiple resolutions works in action using the
+    `Multiple Resolutions and Aspect Ratios demo project <https://github.com/godotengine/godot-demo-projects/tree/master/gui/multiple_resolutions>`__.
+
 Base size
 ---------
 
@@ -69,6 +74,14 @@ that are different from this base size. Godot offers many ways to
 control how the viewport will be resized and stretched to different
 screen sizes.
 
+To configure the stretch base size at runtime from a script, use the
+``get_tree().root.content_scale_size`` property (see
+:ref:`Window.content_scale_size <class_Window_property_content_scale_size>`).
+Changing this value can indirectly change the size of 2D elements. However, to
+provide an user-accessible scaling option, using
+:ref:`doc_multiple_resolutions_stretch_scale` is recommended as it's easier to
+adjust.
+
 .. note::
 
    Godot follows a modern approach to multiple resolutions. The engine will
@@ -153,6 +166,11 @@ demonstrate the effect of different stretch modes. A single sprite, also
 
    .. image:: img/stretch_viewport_expand.gif
 
+To configure the stretch mode at runtime from a script, use the
+``get_tree().root.content_scale_mode`` property (see
+:ref:`Window.content_scale_mode <class_Window_property_content_scale_mode>`
+and the :ref:`ContentScaleMode <enum_Window_ContentScaleMode>` enum).
+
 Stretch Aspect
 ^^^^^^^^^^^^^^
 
@@ -230,30 +248,36 @@ to the region outside the blue frame you see in the 2D editor.
     To allow the user to choose their preferred screen orientation at run-time,
     remember to set **Display > Window > Handheld > Orientation** to ``sensor``.
 
-Stretch Shrink
-^^^^^^^^^^^^^^
+To configure the stretch aspect at runtime from a script, use the
+``get_tree().root.content_scale_aspect`` property (see
+:ref:`Window.content_scale_aspect <class_Window_property_content_scale_aspect>`
+and the :ref:`ContentScaleAspect <enum_Window_ContentScaleAspect>` enum).
 
-The **Shrink** setting allows you to add an extra scaling factor on top of
-what the **Stretch** options above already provide. The default value of 1
-means that no scaling occurs.
+.. _doc_multiple_resolutions_stretch_scale:
 
-If, for example, you set **Shrink** to 4 and leave **Stretch Mode** on
-**Disabled**, each unit in your scene will correspond to 4×4 pixels on the
-screen.
+Stretch Scale
+^^^^^^^^^^^^^
 
-If **Stretch Mode** is set to something other than **Disabled**, the size of
-the root viewport is scaled down by the **Shrink** factor, and pixels
-in the output are scaled up by the same amount. This is rarely useful for
-2D games, but can be used to increase performance in 3D games
-by rendering them at a lower resolution.
+The **Scale** setting allows you to add an extra scaling factor on top of
+what the **Stretch** options above already provide. The default value of ``1.0``
+means that no additional scaling occurs.
 
-From scripts
-^^^^^^^^^^^^
+For example, if you set **Scale** to ``2.0`` and leave **Stretch Mode** on
+**Disabled**, each unit in your scene will correspond to 2×2 pixels on the
+screen. This is a good way to provide scaling options for non-game applications.
 
-To configure stretching at runtime from a script, use the
-``get_tree().root.content_scale_mode`` (see
-:ref:`Window.content_scale_mode <class_Window_property_content_scale_mode>`
-and the :ref:`ContentScaleMode <enum_Window_ContentScaleMode>` enum).
+If **Stretch Mode** is set to **canvas_items**, 2D elements will be scaled
+relative to the base window size, then multiplied by the **Scale** setting. This
+can be exposed to players to allow them to adjust the automatically determined
+scale to their liking, for better accessibility.
+
+If **Stretch Mode** is set to **viewport**, the viewport's resolution is divided
+by **Scale**. This makes pixels look larger and reduces rendering resolution
+(with a given window size), which can improve performance.
+
+To configure the stretch scale at runtime from a script, use the
+``get_tree().root.content_scale_factor`` property (see
+:ref:`Window.content_scale_factor <class_Window_property_content_scale_factor>`).
 
 Common use case scenarios
 -------------------------