Update comment from bn.h

doc/crypto/BN_mod_mul_montgomery.pod

@@ -62,8 +62,9 @@ The B<BN_MONT_CTX> structure is defined as follows:
         int ri;         /* number of bits in R */
         BIGNUM RR;      /* R^2 (used to convert to Montgomery form) */
         BIGNUM N;       /* The modulus */
-        BIGNUM Ni;      /* The inverse of N (bignum form) */
-        BN_ULONG n0;    /* The inverse of N in word form */
+        BIGNUM Ni;      /* R*(1/R mod N) - N*Ni = 1
+                         * (Ni is only stored for bignum algorithm) */
+        BN_ULONG n0;    /* least significant word of Ni */
         int flags;
         } BN_MONT_CTX;