Document ASN1_INTEGER_get_uint64(3), ASN1_INTEGER_get_int64(3),

Document ASN1_INTEGER_get_uint64(3), ASN1_INTEGER_get_int64(3),
ASN1_INTEGER_set_uint64(3), ASN1_INTEGER_set_int64(3),
ASN1_ENUMERATED_get_int64(3), and ASN1_ENUMERATED_set_int64(3)
recently provided by tb@.

Even though Dr. Steven Henson also documented these functions in OpenSSL,
the text over there is excessively verbose, repetitive, very badly ordered,
and incomplete, so i chose to instead write this patch from scratch,
also adding some precision in a few places.