Document what EXFLAG_SET is for in x509v3.h

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

include/openssl/x509v3.h

@@ -355,6 +355,7 @@ struct ISSUING_DIST_POINT_st {
 # define EXFLAG_SI               0x20
 # define EXFLAG_V1               0x40
 # define EXFLAG_INVALID          0x80
+/* EXFLAG_SET is set to indicate that some values have been precomputed */
 # define EXFLAG_SET              0x100
 # define EXFLAG_CRITICAL         0x200
 # define EXFLAG_PROXY            0x400