if attestations are going to be uploaded, do it before completion of file upload session

Author: ewdurbin

peps/pep-0694.rst

@@ -777,6 +777,8 @@ Servers **MAY** support uploading of digital attestations for files (see :pep:`7
 This support will be indicated by inclusion of an ``attestations_url`` key in the
 ``http-post-application-octet-stream`` map of the ``mechanism`` map of the
 :ref:`file upload session creation response body <file-upload-session-response>`.
+Attestations **MUST** be uploaded to the ``attestations_url`` before
+:ref:`file upload session completion <file-upload-session-completion>`.
 
 To upload an attestation, a client submits a ``POST`` request to the ``attestations_url``
 containing a JSON array of :pep:`attestation objects <740#attestation-objects>` like: