Add EC key generation paragraph in doc/HOWTO/keys.txt

Seems this documentation is not dead, so add this missing part

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4037)

(cherry picked from commit 003ef7ef)