Hdrp/docs/glossary f number (#6523)

* Update Glossary.md

* Update Glossary.md

Author: JordanL8

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

@@ -40,7 +40,7 @@ A face refers to one side of a piece of geometry. The front face is the side of
 
 #### f-number:
 
-The ratio of the focal length to the diameter of the camera lens.
+The ratio of the focal length to the diameter of the camera lens. HDRP technically uses [t-number](https://en.wikipedia.org/wiki/F-number#T-stop), but since Cameras in Unity are optically perfect, f-number and t-number are identical.
 
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