[NFC][flang][OpenMP] Add comment for handling the data race problem in copyin clause

[peixin]

Summary

The reference and description is missed before.

[kiranchandramohan]

LGTM.
Nit: Add NFC to the title.

flang/lib/Lower/OpenMP.cpp

if (const auto &copyinClause =
           TODO(converter.getCurrentLocation(),
                "pointer or allocatable variables in Copyin clause");
         assert(sym->has<Fortran::semantics::HostAssocDetails>() &&
                "No host-association found");
         converter.copyHostAssociateVar(*sym);
       }
     }
   }
+  // [OMP 5.0, 2.19.6.1] The copy is done after the team is formed and prior to
+  // the execution of the associated structured block. Emit implicit barrier to
+  // synchronize threads and avoid data races on propagation master's thread
+  // values of threadprivate variables to local instances of that variables of
+  // all other implicit threads.
   if (hasCopyin)
     firOpBuilder.create<mlir::omp::BarrierOp>(converter.getCurrentLocation());
   firOpBuilder.restoreInsertionPoint(insPt);
 }
 
 static void genObjectList(const Fortran::parser::OmpObjectList &objectList,
                           Fortran::lower::AbstractConverter &converter,
                           llvm::SmallVectorImpl<Value> &operands) {