Fix ECDSA_SIG docs

They incorrectly said that i2d_ECDSA_SIG returns 0 on error. In fact it
returns a negative value on error.

We fix this by moving the i2d_ECDSA_SIG/d2i_ECDSA_SIG docs onto the same
page as all the other d2i/i2d docs.

Fixes #9517

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/9533)

(cherry picked from commit bbda79976b5c5095c5e6557311c86c623ba335f1)