Man pages matching name pkcs7_sign_add_signer in section 3ssl:

PKCS7_sign_add_signer.3ssl.en	debian 10 / libssl-doc
PKCS7_sign_add_signer.3ssl.en	debian 11 / libssl-doc
PKCS7_sign_add_signer.3ssl.en	debian 12 / libssl-doc