propose a implementation defined warning mechanism

Hello all,

seems like an oversight to not have any concept of warnings in XProc.

I would propose adding a implementation defined warning, not unlike
many existing specs have.

in section 6 Errors add (either 6.3 or top level) ;

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An XProc implementation may also raise warnings at any time during
XProc processing. The circumstances in which warnings are generated
and managed are implementation defined.

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will also need to add some words here  A.1 Implementation-defined features

cheers, Jim Fuller