I have reviewed this document as part of the security directorate's 
ongoing effort to review all IETF documents being processed by the 
IESG.  These comments were written primarily for the benefit of the 
security area directors.  Document editors and WG chairs should treat 
these comments just like any other last call comments.


This document describes a KeyPurposeId for document signing.  As written, the document is straightforward and on point.
I only have the following editorial nits.

NIT:
Introduction: 2nd paragraph first sentence may have extraneous "also"?
The paragraph is describing when both code signing and S/MIME are used
 how adverse affects can occur; but given the long length of the sentence
it reads awkwardly.

Section 6: this may be redundant (or rhetorical) but worth mentioning. Ass this is 
now a new id-kp for document signing (e.g. id-kp-documentSigning), would it be an error
Or a "no op" if there was the presence now of the combination of the old use
e.g. id-kp-emailProtection, id-kp-codSigning and id-kp-documentSigning?  
I don't think is further breaks or worsens current security considerations, but its handling
And expected behavior may be worth a mention.