Explain the start and process functions for particle shaders

Add references in the particle shader doc and the migration guide

tutorials/migrating/upgrading_to_godot_4.rst

@@ -505,14 +505,12 @@ Godot 3.x, you will have to change its code to call :ref:`class_RenderingServer`
 methods that affect environment effects' quality. Only the "base" toggle of each
 environment effect and its visual knobs remain within the Environment resource.
 
-Updating external shaders
-^^^^^^^^^^^^^^^^^^^^^^^^^
+Updating shaders
+^^^^^^^^^^^^^^^^
 
-**Only shaders that are built-in to a scene file are modified by the project
-upgrade tool.** This means external shaders (saved to ``.gdshader`` files)
-need to be updated manually.
+There have been some changes to shaders that aren't covered by the upgrade tool.
 
-The ``.shader`` file extension is also no longer supported, which means you must
+The ``.shader`` file extension is no longer supported, which means you must
 rename ``.shader`` files to ``.gdshader`` and update references accordingly in
 scene/resource files using an external text editor.
 
@@ -521,7 +519,9 @@ Some notable renames you will need to perform in shaders are:
 - Texture filter and repeat modes are now set on individual uniforms, rather
   than the texture files themselves.
 - ``hint_albedo`` is now ``source_color``.
-- :ref:`Projection matrix variables were renamed. <doc_spatial_shader>`
+- :ref:`Built in matrix variables were renamed. <doc_spatial_shader>`
+- Particles shaders no longer use the ``vertex()`` processor function. Instead
+  they use ``start()`` and ``process()``.
 
 See :ref:`doc_shading_language` for more information.
 

tutorials/shaders/shader_reference/particle_shader.rst

@@ -3,22 +3,19 @@
 Particle shaders
 ================
 
-Particle shaders are a special type of vertex shader that runs before the
-object is drawn. They are used for calculating material properties such as
-color, position, and rotation. They are drawn with any regular material for
-CanvasItem or Spatial, depending on whether they are 2D or 3D.
-
-Particle shaders are unique because they are not used to draw the object
-itself; they are used to calculate particle properties, which are then used
-by the CanvasItem of Spatial shader. They contain only a vertex processor
-function that outputs multiple properties (see built-ins below).
-
-Particle shaders use a transform feedback shader, which is a special type of
-vertex shader that runs on its own. It takes in data in a buffer like a regular
-vertex shader does, but it also outputs to data buffers instead of outputting
-to the fragment shader for pixel-processing. Because of this, transform feedback
-shaders can build on themselves each run, unlike other shaders that discard the
-data they have calculated once they draw to the frame buffer.
+Particle shaders are a special type of shader that runs before the object is
+drawn. They are used for calculating material properties such as color,
+position, and rotation. They are drawn with any regular material for CanvasItem
+or Spatial, depending on whether they are 2D or 3D.
+
+Particle shaders are unique because they are not used to draw the object itself;
+they are used to calculate particle properties, which are then used by the
+CanvasItem of Spatial shader. They contain two processor functions: ``start()``
+and ``process()``.
+
+Unlike other shader types, particle shaders keep the data that was output the
+previous frame. Therefore, particle shaders ca be used for complex effects that
+take place over multiple frames.
 
 .. note::