message: Update misleading docstring references to subtraction for assign and add ops.
diff: @@ -670,7 +670,7 @@ class Variable(six.with_metaclass(VariableMetaclass,
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered addition has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -687,7 +687,7 @@ class Variable(six.with_metaclass(VariableMetaclass,
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered assignment has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -734,7 +734,7 @@ class Variable(six.with_metaclass(VariableMetaclass,
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered assignment has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -836,7 +836,7 @@ class Variable(six.with_metaclass(VariableMetaclass,
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered addition has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -887,7 +887,7 @@ class Variable(six.with_metaclass(VariableMetaclass,
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered assignment has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -1993,7 +1993,7 @@ class RefVariable(VariableV1):
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered addition has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -2017,7 +2017,7 @@ class RefVariable(VariableV1):
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered assignment has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -2071,7 +2071,7 @@ class RefVariable(VariableV1):
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered assignment has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -2176,7 +2176,7 @@ class RefVariable(VariableV1):
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered addition has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  @@ -2228,7 +2228,7 @@ class RefVariable(VariableV1):
  Returns:
  A `Tensor` that will hold the new value of this variable after
  - the scattered subtraction has completed.
  + the scattered assignment has completed.
  Raises:
  ValueError: if `sparse_delta` is not an `IndexedSlices`.
  