[mlir][Standard][NFC] Fix op documentation

[jsetoain]

Summary

A couple of standard op examples that use an outdated syntax need an
update.

[rriddle]

Nice, thanks!

[jsetoain]

I can't land this myself, please go ahead when ready!

mlir/include/mlir/Dialect/StandardOps/IR/Ops.td

let description = [{
 
     Note: the semantics of division by zero or signed division overflow (minimum
     value divided by -1) is TBD; do NOT assume any specific behavior.
 
     Example:
 
     ```mlir
     // Scalar signed integer division.
-    %a = divis %b, %c : i64
+    %a = divi_signed %b, %c : i64
 
     // SIMD vector element-wise division.
-    %f = divis %g, %h : vector<4xi32>
+    %f = divi_signed %g, %h : vector<4xi32>
 
     // Tensor element-wise integer division.
-    %x = divis %y, %z : tensor<4x?xi8>
+    %x = divi_signed %y, %z : tensor<4x?xi8>
     ```
   }];
   let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // SignedFloorDivIOp
 //===----------------------------------------------------------------------===//
let description = [{
 
     Note: the semantics of division by zero is TBD; do NOT assume any specific
     behavior.
 
     Example:
 
     ```mlir
     // Scalar signed integer division remainder.
-    %a = remis %b, %c : i64
+    %a = remi_signed %b, %c : i64
 
     // SIMD vector element-wise division remainder.
-    %f = remis %g, %h : vector<4xi32>
+    %f = remi_signed %g, %h : vector<4xi32>
 
     // Tensor element-wise integer division remainder.
-    %x = remis %y, %z : tensor<4x?xi8>
+    %x = remi_signed %y, %z : tensor<4x?xi8>
     ```
   }];
   let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // SignedShiftRightOp
 //===----------------------------------------------------------------------===//
let description = [{
 
     Note: the semantics of division by zero is TBD; do NOT assume any specific
     behavior.
 
     Example:
 
     ```mlir
     // Scalar unsigned integer division.
-    %a = diviu %b, %c : i64
+    %a = divi_unsigned %b, %c : i64
 
     // SIMD vector element-wise division.
-    %f = diviu %g, %h : vector<4xi32>
+    %f = divi_unsigned %g, %h : vector<4xi32>
 
     // Tensor element-wise integer division.
-    %x = diviu %y, %z : tensor<4x?xi8>
+    %x = divi_unsigned %y, %z : tensor<4x?xi8>
     ```
   }];
   let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // UnsignedRemIOp
 //===----------------------------------------------------------------------===//
let description = [{
 
     Note: the semantics of division by zero is TBD; do NOT assume any specific
     behavior.
 
     Example:
 
     ```mlir
     // Scalar unsigned integer division remainder.
-    %a = remiu %b, %c : i64
+    %a = remi_unsigned %b, %c : i64
 
     // SIMD vector element-wise division remainder.
-    %f = remiu %g, %h : vector<4xi32>
+    %f = remi_unsigned %g, %h : vector<4xi32>
 
     // Tensor element-wise integer division remainder.
-    %x = remiu %y, %z : tensor<4x?xi8>
+    %x = remi_unsigned %y, %z : tensor<4x?xi8>
     ```
   }];
   let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // UnsignedShiftRightOp
 //===----------------------------------------------------------------------===//