Do not call the 'UploadStashFile' hook for partially uploaded files

Do not call the 'UploadStashFile' hook for partially uploaded files

Consumers of this hook don't expect partial files, and even if they did,
they have no way to tell if they've been given a partial file.

Also document a pre-existing restriction that verifyUpload() must be
called first (for non-partial files). ApiUpload previously called this
function for partial files too, causing T143161.

Follow-up to 2ee27d9a4de163dc3a2b752823a04f5338005627.

Bug: T143161
Change-Id: I107cb957e046545ea72834d72233cc1ed2867e42