Improve top warning in dhm.h
* primes cannot be trusted because of the possibility of backdoors
* that can't be effectively checked for.
*
+ * Diffie-Hellman-Merkle is therefore a security risk when not using
+ * standardized primes generated using a trustworthy ("nothing up
+ * my sleeve") method, such as the RFC 3526 / 7919 primes. In the TLS
+ * protocol, DH parameters need to be negotiated, so using the default
+ * primes systematically is not always an option. If possible, use
+ * Elliptic Curve Diffie-Hellman (ECDH), which has better performance,
+ * and for which the TLS protocol mandates the use of standard
+ * parameters that were generated in a nothing-up-my-sleeve manner.
* We therefore consider DHE a security risk. If possible, it is
* recommended users should consider preferring other methods of
* key exchange.
