Fix documentation referring to 'function code'
Fix documentation referring to 'function code'

ERR_GET_FUNC was removed, so remove references to 'function code' as
well from docs.

CLA: trivial

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Dmitry Belyavskiy <38c64d6e24766247aad56af3a6ddb0056ad36a44@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16353)

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