mention ! in semantics description

Author: jekbradbury

docs/source/semantics.rst

@@ -43,6 +43,8 @@ There is only one reduction operator for the entire statement because
 combinations like :code:`max/+` on different dimensions have different mathematical meanings depending on loop order.
 All reduction operators are considered to be associative and commutative to
 allow for arbitrary order of evaluation.
+Reduction operators may be suffixed with :code:`!` to indicate that the tensor to which values are accumulated should first be initialized with the identity of the reduction operator (e.g., zero for :code:`+`).
+Otherwise, values are accumulated directly to the output or temporary tensor passed to the kernel.
 
 Size Expressions
 ----------------
@@ -69,7 +71,8 @@ evaluating :code:`rhs_expression` at all points in the iteration space defined
 by the ranges of the loop index variables and reducing into the entry of the
 tensor specified on the left-hand side. The order in which these expressions
 are evaluated should not change the result because the reduction is
-associative and commutative.
+associative and commutative. If :code:`!` is not present, :code:`v` is not
+re-initialized, and the reduction takes into account the existing values in :code:`v`.
 
 Expressions
 -----------