[HDRP] Adding limitations of decal emissive positive contribution to the documentation (#5237)

* Add emissive positive contribution limitation to the doc

* changelog

* Update CHANGELOG.md

* Update Decal-Projector.md

* Update Decal-Shader.md

Co-authored-by: sebastienlagarde <[email protected]>

Author: remi-chapelain

com.unity.render-pipelines.high-definition/Documentation~/Decal-Projector.md

@@ -55,7 +55,8 @@ Using the Inspector allows you to change all of the Decal Projector properties,
 ## Limitations
 
 - The Decal Projector can affect opaque Materials with either a [Decal Shader](Decal-Shader.md) or a [Decal Master Stack](master-stack-decal.md). However, it can only affect transparent Materials with the [Decal Shader](Decal-Shader.md).
-- Decal Emissive isn't supported on Transparent Material.
+- Emissive decals isn't supported on Transparent Material.
+- Emissive decals always give an additive positive contribution. This property does not affect the existing emissive properties of the Materials assigned to a GameObject.
 - The **Receive Decals** property of Materials in HDRP does not affect emissive decals. HDRP always renders emissive decals unless you use Decal Layers, which can disable emissive decals on a Layer by Layer basis.
 - If you project a decal onto a transparent surface, HDRP ignores the decal's Texture tiling.
 - In **Project Settings > Graphics**, if **Instancing Variants** is set to **Strip All**, Unity strips the Decal Shader this component references when you build your Project. This happens even if you include the Shader in the **Always Included Shaders** list. If Unity strips the Shader during the build process, the decal does not appear in your built Application.

com.unity.render-pipelines.high-definition/Documentation~/Decal-Shader.md

@@ -15,7 +15,7 @@ These properties allow you to set the affected attributes of the Material the de
 | **Affect Metal**                | Enable the checkbox to make the decal use the metallic property of its **Mask Map**. Otherwise the decal has no metallic effect. Uses the red channel of the **Mask Map**.<br />This property only appears when you enable the **Metal and Ambient Occlusion properties** checkbox in your [HDRP Asset](HDRP-Asset.md#Decals). |
 | **Affect Ambient Occlusion**    | Enable the checkbox to make the decal use the ambient occlusion property of its **Mask Map**. Otherwise the decal has no ambient occlusion effect. Uses the green channel of the **Mask Map**.<br />This property only appears when you enable the **Metal and Ambient Occlusion properties** checkbox in your [HDRP Asset](HDRP-Asset.md#Decals). |
 | **Affect Smoothness**           | Enable the checkbox to make the decal use the smoothness property of its **Mask Map**. Otherwise the decal has no smoothness effect. Uses the alpha channel of the **Mask Map**.<br /> |
-| **Affect Emissive**             | Enable the checkbox to make this decal emissive. When enabled, this Material appears self-illuminated and acts as a visible source of light. This property does not work with transparent receiving Materials. |
+| **Affect Emissive**             | Enable the checkbox to make this decal emissive. When enabled, this Material appears self-illuminated and acts as a visible source of light. This property does not work with transparent receiving Materials. Emissive decals always give an additive positive contribution. This property does not affect the existing emissive properties of the Materials assigned to a GameObject. |
 
 
 ### Surface Inputs