gallium/docs: document TGSI_SEMANTIC_EDGEFLAG

src/gallium/docs/source/tgsi.rst

@@ -1397,7 +1397,15 @@ back-facing.
 TGSI_SEMANTIC_EDGEFLAG
 """"""""""""""""""""""
 
-XXX no clue
+For vertex shaders, this sematic label indicates that an input or
+output is a boolean edge flag.  The register layout is [F, x, x, x]
+where F is 0.0 or 1.0 and x = don't care.  Normally, the vertex shader
+simply copies the edge flag input to the edgeflag output.
+
+Edge flags are used to control which lines or points are actually
+drawn when the polygon mode converts triangles/quads/polygons into
+points or lines.
+
 
 
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