Make the Vulkan SDK note more prominent in Compiling for macOS

`important` in Sphinx is green, but `warning` is yellow, so it's
more noticeable.

development/compiling/compiling_for_macos.rst

@@ -15,10 +15,12 @@ For compiling under macOS, the following is required:
 - `Xcode <https://apps.apple.com/us/app/xcode/id497799835>`_
   (or the more lightweight Command Line Tools for Xcode).
 
-.. important::
+.. warning::
 
     If you are building the ``master`` branch, download and install the
-    `Vulkan SDK for macOS <https://vulkan.lunarg.com/sdk/home>`__.
+    `Vulkan SDK for macOS <https://vulkan.lunarg.com/sdk/home>`__. This
+    is **required** to compile Godot 4.x, as MoltenVK is used to translate Vulkan
+    to Metal (macOS doesn't support Vulkan out of the box).
 
 .. note:: If you have `Homebrew <https://brew.sh/>`_ installed, you can easily
           install SCons using the following command::