[url] clarify "scheme specific definitions" problem (#34)

Also from Bjoern http://www.ietf.org/mail-archive/web/apps-discuss/current/msg13507.html

The 'problem' is that you can't 'simply require somebody' to update specific scheme specifications. I think some of the possible outcomes will make revision of scheme definitions easier or
> harder. As part of due diligence, it may be necessary to review existing scheme definitions to see if they need update when updating 3986 or 3987,

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Reply to this email directly or view it on GitHub:
https://github.com/webspecs/url/issues/34