clarify "scheme specific definitions" problem

[masinter]

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,