[flang][NFC] Update comments.

[clementval]

I think you can commit this kind of change without a review.

flang/include/flang/Optimizer/Dialect/FIRType.h

 //===-- Optimizer/Dialect/FIRType.h -- FIR types ----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+//
+// Coding style: https://mlir.llvm.org/getting_started/DeveloperGuide/
+//
+//===----------------------------------------------------------------------===//
 
 #ifndef OPTIMIZER_DIALECT_FIRTYPE_H
 #define OPTIMIZER_DIALECT_FIRTYPE_H
 
 #include "mlir/IR/BuiltinAttributes.h"
 #include "mlir/IR/BuiltinTypes.h"
 #include "llvm/ADT/SmallVector.h"
 

 bool isa_std_type(mlir::Type t);
 
 /// Is `t` any of the FIR dialect or Standard dialect types?
 bool isa_fir_or_std_type(mlir::Type t);
 
 /// Is `t` a FIR dialect type that implies a memory (de)reference?
 bool isa_ref_type(mlir::Type t);
 
-/// Is `t` a type that is always trivially pass-by-reference?
+/// Is `t` a type that is always trivially pass-by-reference? Specifically, this
+/// is testing if `t` is a ReferenceType or any box type. Compare this to
+/// conformsWithPassByRef(), which includes pointers and allocatables.
 bool isa_passbyref_type(mlir::Type t);
 
 /// Is `t` a boxed type?
 bool isa_box_type(mlir::Type t);
 
 /// Is `t` a type that can conform to be pass-by-reference? Depending on the
 /// context, these types may simply demote to pass-by-reference or a reference
 /// to them may have to be passed instead.