Update comments.

Author: lntue

libc/config/config.json

@@ -90,7 +90,7 @@
     },
     "LIBC_CONF_FREXP_INF_NAN_EXPONENT": {
       "value": "",
-      "doc": "Set the specific exp value for Inf/NaN inputs."
+      "doc": "The value written back to the second parameter when calling frexp/frexpf/frexpl` with `+/-Inf`/`NaN` is unspecified.  Configue an explicit exp value for Inf/NaN inputs."
     }
   },
   "qsort": {

libc/src/__support/FPUtil/ManipulationFunctions.h

@@ -33,6 +33,10 @@ LIBC_INLINE T frexp(T x, int &exp) {
   FPBits<T> bits(x);
   if (bits.is_inf_or_nan()) {
 #ifdef LIBC_FREXP_INF_NAN_EXPONENT
+    // The value written back to the second parameter when calling
+    // frexp/frexpf/frexpl` with `+/-Inf`/`NaN` is unspecified in the standard.
+    // Set the exp value for Inf/NaN inputs explicitly to
+    // LIBC_FREXP_INF_NAN_EXPONENT if it is defined.
     exp = LIBC_FREXP_INF_NAN_EXPONENT;
 #endif // LIBC_FREXP_INF_NAN_EXPONENT
     return x;