Update ui_introduction_to_the_ui_system.rst

Not all stretch modes of TextureRect was described.

getting_started/step_by_step/ui_introduction_to_the_ui_system.rst

@@ -76,12 +76,14 @@ TextureRect
 It seems similar to the Sprite node but it offers multiple scaling modes.
 Set the Stretch Mode property to change its behaviour:
 
--  ``Tile`` makes the texture repeat, but it won't scale
--  The default, ``Scale and Expand (compat)``, lets you scale the
-   texture up, but it forces a minimal size
+- ``Scale On Expand (compat)`` scales the texture to fit the node’s bounding rectangle, only if ``expand`` property is ``true``; otherwise, it behaves like ``Keep`` mode. Default mode for backwards compatibility.
+- ``Sacle`` scales the texture to fit the node’s bounding rectangle
+- ``Tile`` makes the texture repeat, but it won't scale
 -  ``Keep`` and ``Keep Centered`` force the texture to remain at its
    original size, in the top left corner or the center of the frame
    respectively
+- ``Keep Aspect`` and ``Keep Aspect Centered`` scales the texture but force it to remain its original aspect ratio, in the top left corner or the center of the frame respectively
+- ``Keep Aspect Covered`` works just like ``Keep Aspect Centered`` but the shorter side fits the bounding rectangle and the other one clips to the node’s limits
 
 As with Sprite nodes, you can modulate the TextureRect's colour. Click
 the ``Modulate`` property and use the color picker.