Fix EC_KEY_set_private_key() NULL priv_key docs
Fix EC_KEY_set_private_key() NULL priv_key docs

Updates the docs to describe EC_KEY_set_private_key() function behavior
when a NULL priv_key argument is passed.

Reviewed-by: Nicola Tuveri <5a38b4ac03c90be61ead22c1d11b6e9f49425984@gmail.com>
Reviewed-by: Todd Short <0267f0ecbdb3e99cdfd94eb139bf30919fa04d2f@me.com>
(Merged from https://github.com/openssl/openssl/pull/18942)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl