Emphasize the importance of mipmaps for textureLod() in Screen-reading shaders

This also mentions that reading the normal/roughness buffer is only
currently supported in Forward+.

tutorials/shaders/custom_postprocessing.rst

@@ -40,8 +40,9 @@ Your scene tree will look something like this:
 
     As of the time of writing, Godot does not support rendering to multiple
     buffers at the same time. Your post-processing shader will not have access
-    to normals or other render passes. You only have access to the rendered
-    frame.
+    to other render passes and buffers not exposed by Godot (such as depth or
+    normal/roughness). You only have access to the rendered frame and buffers
+    exposed by Godot as samplers.
 
 For this demo, we will use this :ref:`Sprite <class_Sprite2D>` of a sheep.
 
@@ -60,6 +61,8 @@ shader by `arlez80 <https://bitbucket.org/arlez80/hex-mosaic/src/master/>`_,
     shader_type canvas_item;
 
     uniform vec2 size = vec2(32.0, 28.0);
+    // If you intend to read from mipmaps with `textureLod()` LOD values greater than `0.0`,
+    // use `filter_nearest_mipmap` instead. This shader doesn't require it.
     uniform sampler2D screen_texture : hint_screen_texture, repeat_disable, filter_nearest;
 
     void fragment() {