Added comment

Author: eugeneepshteyn

flang/include/flang/Evaluate/real.h

@@ -490,6 +490,9 @@ template <typename WORD, int PREC> class Real {
       bool isNegative, int exponent, const Fraction &, Rounding, RoundingBits,
       bool multiply = false);
 
+  // Require alignment, in case code generation code on x86_64 decides that
+  // our Real object is suitable for SSE2 instructions and then gets surprised
+  // by unaligned address.
   alignas(Word::alignment / 8) Word word_{}; // an Integer<>
 };