update DSA docs

The B<DSA_PUBKEY> functions should be used in preference to the B<DSAPublicKey>

functions when encoding public keys because they use a standard format.

The B<DSAPublicKey> functions use an non standard format the actual data encoded

depends on the value of the B<write_params> field of the B<a> key parameter.

If B<write_params> is zero then only the B<pub_key> field is encoded as an

B<INTEGER>. If B<write_params> is 1 then a B<SEQUENCE> consisting of the

B<p>, B<q>, B<g> and B<pub_key> respectively fields are encoded.

The B<DSAPublicKey> functions use a non standard format which is a

B<SEQUENCE> consisting of the B<p>, B<q>, B<g> and B<pub_key> fields

respectively.

The B<DSAPrivateKey> functions also use a non standard structure consisting

consisting of a SEQUENCE containing the B<p>, B<q>, B<g> and B<pub_key> and