Fix error behavior for initialize_to_identity

Cherry pick KhronosGroup#651 from main
(cherry picked from commit 8424248)

adoc/chapters/programming_interface.adoc

@@ -13885,9 +13885,10 @@ property::reduction::initialize_to_identity
       identity value passed to the reduction interface, or to the identity
       value determined by the [code]#known_identity# trait if no identity value
       was specified. If no identity value was specified and an identity value
-      cannot be determined by the [code]#known_identity# trait, the compiler
-      must raise a diagnostic. When this property is set, the original value of
-      the reduction variable is not included in the reduction.
+      cannot be determined by the [code]#known_identity# trait, the
+      implementation must throw an [code]#exception# with the
+      [code]#errc::invalid# error code. When this property is set, the original
+      value of the reduction variable is not included in the reduction.
 
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