Clarified that the input box cannot be absent.

I do not want to identify later whether the box might be absent
and generate redundant present checks. To avoid redundant present
checks I can add attribute like `present`/`maybe_absent`,
but I think it should be easy to generate these ops under
the present check upstream. So I would like to make them
simple at least for now.

Author: vzakhari

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -3422,6 +3422,8 @@ def fir_IsContiguousBoxOp : fir_Op<"is_contiguous_box", [NoMemoryEffect]> {
     Returns true iff the boxed entity is contiguous:
       * in the leading dimension (if `innermost` attribute is set),
       * in all dimensions (if `innermost` attribute is not set).
+
+    The input box cannot be absent.
   }];
   let arguments = (ins AnyBoxLike:$box, UnitAttr:$innermost);
   let results = (outs I1);
@@ -3435,6 +3437,10 @@ def fir_IsContiguousBoxOp : fir_Op<"is_contiguous_box", [NoMemoryEffect]> {
 def fir_BoxTotalElementsOp
     : fir_SimpleOneResultOp<"box_total_elements", [NoMemoryEffect]> {
   let summary = "Returns the boxed entity's total size in elements";
+  let description = [{
+    Returns the boxed entity's total size in elements.
+    The input box cannot be absent.
+  }];
   let arguments = (ins AnyBoxLike:$box);
   let results = (outs AnyIntegerLike);
   let hasCanonicalizer = 1;