NFC: Callout restriction on folding 0-result ops in documentation.

[• alihanima]

This comment was removed by mehdi_amini.

mlir/docs/Canonicalization.md

 ///  2. They can mutate the operation in place, without changing anything else
 ///     in the IR. In this case, return success.
 ///  3. They can return a list of existing values or attribute that can be used
 ///     instead of the operation. In this case, fill in the results list and
 ///     return success. The results list must correspond 1-1 with the results of
 ///     the operation, partial folding is not supported. The caller will remove
 ///     the operation and use those results instead.
 ///
+/// Note that this mechanism cannot be used to remove 0-result operations.
 LogicalResult MyOp::fold(ArrayRef<Attribute> operands,
                          SmallVectorImpl<OpFoldResult> &results) {
   ...
 }
 ```
 
 In the above, for each method an `ArrayRef<Attribute>` is provided that
 corresponds to the constant attribute value of each of the operands. These