Document a workaround in the render pipeline code

I originally wanted to get of the legacy IVideoDriver::setRenderTarget and this
workaround altogether, but that ended up being too much work.
Instead I'm documenting my findings so they're not wasted.

src/client/render/pipeline.cpp

@@ -179,6 +179,22 @@ void TextureBufferOutput::activate(PipelineContext &context)
 
 	// Use legacy call when there's single texture without depth texture
 	// This binds default depth buffer to the FBO
+
+	// This is currently neccessary for a working depth buffer in the following cases:
+	// - post-processing disabled, undersampling enabled
+	//   (addUpscaling specifies no depth texture)
+	// - post-processing disabled, 3d_mode = sidebyside / topbottom / crossview
+	//	 (populateSideBySidePipeline specifies no depth texture)
+	// - post-processing disabled, 3d_mode = interlaced (probably)
+	//   (populateInterlacedPipeline specifies no depth texture)
+	// With post-processing disabled, the world is rendered to the TextureBufferOutput
+	// created in the functions listed above, so a depth buffer is needed.
+	// With post-processing enabled, only a fullscreen rectangle is rendered to
+	// this TextureBufferOutput, so a depth buffer isn't actually needed.
+	// But: These pipeline steps shouldn't rely on what ends up being rendered to
+	// the TextureBufferOutput they provide, since that may change.
+	// https://irc.minetest.net/minetest-dev/2022-10-04#i_6021940
+
 	if (textures.size() == 1 && depth_stencil == NO_DEPTH_TEXTURE) {
 		driver->setRenderTarget(textures[0], m_clear, m_clear, context.clear_color);
 		return;