Add a §4 note making the implicit invariant explicit: the ':' and '/'
delimiters are both members of nss-char (':' is an RFC pchar, '/' the
additionally-allowed NSS character). Notes the corollary that an element
may contain the other delimiter — the source of the mixed-delimiter
round-trip case in §5.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>