Document what EXFLAG_SET is for in x509v3.h
Document what EXFLAG_SET is for in x509v3.h

Reviewed-by: Kurt Roeckx <bb87b47479d83cec3c76132206933257ded727b2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2314)
openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl