Update BulbDrawLitApplicationSurface.md

docs/22.0/BulbDrawLitApplicationSurface.md

@@ -16,6 +16,8 @@
 |`[textureFiltering]`|boolean |Whether to use texture filtering when drawing the application surface. If not specified, the texture filter setting will not be changed|
 |`[alphaBlend]`      |boolean |Whether to use texture filtering when drawing the application surface. If not specified, alpha blending will be disabled               |
 
+Bulb uses gamma correct lighting and, as such, works in a slightly different way to most other GameMaker lighting systems. Instead of multiplying the light on top of a source surface (typically the application surface), Bulb instead combines the source surface with the lighting in a special shader called a "tonemapping" shader. This process is complex, especially for HDR lighting and bloom, so it is wrapped up inside this helper function.
+
 This function should typically be called in the Post Draw event as a replacement for native automatic drawing or other manual drawing (`draw_surface_stretched(application_surface, ...)` etc.) of the application surface.
 
 ?> You'll probably want to call `application_surface_draw_enable(false)` at the start of your game if you're using this function.