Clarify BN_mod_exp docs

Specifically this is not supported with an even modulus and
BN_FLG_CONSTTIME.

Fixes openssl#5082

doc/man3/BN_add.pod

@@ -92,7 +92,9 @@ BN_exp() raises I<a> to the I<p>-th power and places the result in I<r>
 BN_mul().
 
 BN_mod_exp() computes I<a> to the I<p>-th power modulo I<m> (C<r=a^p %
-m>). This function uses less time and space than BN_exp().
+m>). This function uses less time and space than BN_exp(). Note that calling
+this function with an even modulus and when any of B<a>, B<p> or B<m> have the
+BN_FLG_CONSTTIME flag set is not supported.
 
 BN_gcd() computes the greatest common divisor of I<a> and I<b> and
 places the result in I<r>. I<r> may be the same B<BIGNUM> as I<a> or