[HDRP] Update custom pass doc (decals not supported by DrawRenderers custom pass) (#6858)

* update custom pass doc

* Update Custom-Pass-Creating.md

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

Author: alelievr

com.unity.render-pipelines.high-definition/Documentation~/Custom-Pass-Creating.md

@@ -159,6 +159,8 @@ However, not all materials are supported by every injection point in a draw rend
 
 When Unity renders a material that is not supported by the current injection point, it results in an undefined behavior. For example, rendering GameObjects with lit shaders in the **After Opaque Depth And Normal** injection point produces unexpected results.
 
+Note: HDRP does not support decals on GameObjects rendered in the DrawRenderers pass.
+
 <a name="Object-ID-Custom-Pass"></a>
 
 ## Object ID Custom Pass