Restore one more missing line.

extensions/ARB/ARB_gl_spirv.txt

diff --git a/extensions/ARB/ARB_gl_spirv.txt b/extensions/ARB/ARB_gl_spirv.txt
index 3931bd5..56c3c0a 100644
--- a/extensions/ARB/ARB_gl_spirv.txt
+++ b/extensions/ARB/ARB_gl_spirv.txt
@@ -358,7 +358,7 @@
       NA                    -> OpAtomicCompareExchangeWeak
 
       atomicCounterIncrement -> OpAtomicIIncrement
-      atomicCounterDecrement -> OpAtomicIDecrement
+      atomicCounterDecrement -> OpAtomicIDecrement (with post decrement)
       atomicCounter          -> OpAtomicLoad
 
     Mapping of other instructions