docs: Explain TGSI_SEMANTIC_NORMAL.

diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
index 492a1b7..ebee490 100644 (file)
--- a/src/gallium/docs/source/tgsi.rst
+++ b/src/gallium/docs/source/tgsi.rst
@@ -1253,7 +1253,8 @@ can be stored inside a four-component vector.
 TGSI_SEMANTIC_NORMAL
 """"""""""""""""""""
 
-XXX no clue.
+Vertex normal; could be used to implement per-pixel lighting for legacy APIs
+that allow mixing fixed-function and programmable stages.
 
 TGSI_SEMANTIC_FACE
 """"""""""""""""""