DOCG-4889, 5178, 5181 - Fix shader docs issues

Fix various issues in shader docs.

Jira ticket: 
- https://jira.unity3d.com/browse/DOCG-5178
- https://jira.unity3d.com/browse/DOCG-4889
- https://jira.unity3d.com/browse/DOCG-5181

Author: markg-unity

Packages/com.unity.render-pipelines.universal/Documentation~/urp-universal-renderer.md

@@ -100,7 +100,7 @@ With this check box selected, the Renderer processes the Stencil buffer values.
 
 For more information on how Unity works with the Stencil buffer, refer to [ShaderLab: Stencil](https://docs.unity3d.com/Manual/SL-Stencil.html).
 
-In URP, you can use bits 0-15 of the stencil buffer for custom rendering effects.
+In URP, you can use bits 0 to 3 of the stencil buffer for custom rendering effects. This means you can use stencil indices 0 to 15.
 
 ### Compatibility