Man pages matching name pem_read_bio_privatekey in section 3ssl:

PEM_read_bio_PrivateKey.3ssl.en	debian 10 / libssl-doc
PEM_read_bio_PrivateKey.3ssl.en	debian 11 / libssl-doc
PEM_read_bio_PrivateKey.3ssl.en	debian 12 / libssl-doc