GH367: Fix dsa keygen for too-short seed

If the seed value for dsa key generation is too short (< qsize),
return an error. Also update the documentation.

Signed-off-by: Rich Salz <rsalz@akamai.com>
Reviewed-by: Emilia Käsper <emilia@openssl.org>

doc/crypto/DSA_generate_parameters.pod

diff --git a/doc/crypto/DSA_generate_parameters.pod b/doc/crypto/DSA_generate_parameters.pod
index d2a0418..92c89a0 100644
--- a/doc/crypto/DSA_generate_parameters.pod
+++ b/doc/crypto/DSA_generate_parameters.pod
@@ -23,13 +23,12 @@
 DSA_generate_parameters_ex() generates primes p and q and a generator g
 for use in the DSA and stores the result in B<dsa>.
 
-B<bits> is the length of the prime to be generated; the DSS allows a
-maximum of 1024 bits.
+B<bits> is the length of the prime p to be generated.
+For lengths under 2048 bits, the length of q is 160 bits; for lengths
+at least 2048, it is set to 256 bits.
 
-If B<seed> is B<NULL> or B<seed_len> E<lt> 20, the primes will be
-generated at random. Otherwise, the seed is used to generate
-them. If the given seed does not yield a prime q, a new random
-seed is chosen and placed at B<seed>.
+If B<seed> is NULL, the primes will be generated at random.
+If B<seed_len> is less than the length of q, an error is returned.
 
 DSA_generate_parameters_ex() places the iteration count in
 *B<counter_ret> and a counter used for finding a generator in