[MLIR][SCF] Fix LoopPeelOp documentation (NFC) (#113179)

As an example, I added annotations to the peel_front unit test.

```
func.func @loop_peel_first_iter_op() {
  // CHECK: %[[C0:.+]] = arith.constant 0
  // CHECK: %[[C41:.+]] = arith.constant 41
  // CHECK: %[[C5:.+]] = arith.constant 5
  // CHECK: %[[C5_0:.+]] = arith.constant 5
  // CHECK: scf.for %{{.+}} = %[[C0]] to %[[C5_0]] step %[[C5]]
  // CHECK:   arith.addi
  // CHECK: scf.for %{{.+}} = %[[C5_0]] to %[[C41]] step %[[C5]]
  // CHECK:   arith.addi
  %0 = arith.constant 0 : index
  %1 = arith.constant 41 : index
  %2 = arith.constant 5 : index
  scf.for %i = %0 to %1 step %2 {
    arith.addi %i, %i : index
  }
  return
}

module attributes {transform.with_named_sequence} {
  transform.named_sequence @__transform_main(%arg1: !transform.any_op {transform.readonly}) {
    %0 = transform.structured.match ops{["arith.addi"]} in %arg1 : (!transform.any_op) -> !transform.any_op
    %1 = transform.get_parent_op %0 {op_name = "scf.for"} : (!transform.any_op) -> !transform.op<"scf.for">
    %main_loop, %remainder = transform.loop.peel %1 {peel_front = true} : (!transform.op<"scf.for">) -> (!transform.op<"scf.for">, !transform.op<"scf.for">)
    transform.annotate %main_loop "main_loop" : !transform.op<"scf.for">
    transform.annotate %remainder "remainder" : !transform.op<"scf.for">
    transform.yield
  }
}
```
Gives :
```
  func.func @loop_peel_first_iter_op() {
    %c0 = arith.constant 0 : index
    %c41 = arith.constant 41 : index
    %c5 = arith.constant 5 : index
    %c5_0 = arith.constant 5 : index
    scf.for %arg0 = %c0 to %c5_0 step %c5 {
      %0 = arith.addi %arg0, %arg0 : index
    } {remainder}  // The first iteration loop (second result) has been annotated remainder
    scf.for %arg0 = %c5_0 to %c41 step %c5 {
      %0 = arith.addi %arg0, %arg0 : index
    } {main_loop} // The main loop (first result) has been annotated main_loop
    return
  }
```

---------

Co-authored-by: Andrzej Warzyński <andrzej.warzynski@gmail.com>

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