PaceOrderCollectionsByAppModified2

In reviewing the various paces regarding which way collections must be
sorted I found a hole:

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  * PaceAppModified describes using app:modified and also app:revision,
    but no mention of how collections are sorted.

  * PaceAppModified2 is simpler that PaceAppModified, defining
    atom:modified. no mention of sorting.

  * PaceConfigurableCollectionOrdering allows sorting by created, modified,
    or updated dates, and a mechanism for explicitly exposing that sort
    order in the collection. No mention of atom:published, invents
    app:created (without naming it)

  * PaceConfigurableCollectionOrdering2 is a simplified version of
    PaceConfigurableCollectionOrdering, only exposing sorting by
    atom:updated and app:modified

  * RemoveMustOrderCollectionsByUpdated removes the explicit language of
    sorting collections by atom:updated, and implies sort order will remain
    undefined.

  * PaceOrderCollectionsByAppModified wasn't on Sam's list (this was closed
    before the recent in-depth discussions), describes collections being
    sorted by pub:modified (text needs updating for 09 nouns, but concept is
    compatible with 09)
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So ... the options we have available are (1) make sort order undefined, (2)
make sort order variable, (3) leave sort order broken (ie. atom:updated) ...
which leaves out option (4) define one, useful, sort order.

Thus, and in consideration of the *extensive* recent discussions, I've
updated Thomos Broyer's PaceOrderCollectionsByAppModified to be protocol-09
compatible, and also trimmed it to be just the sorting spec text.

In section 9, replace the second paragraph with:

    The entries in the returned Feed MUST be ordered by their
    'xxx:modified' property (Section 8.xx), with the most recently
    modified entries coming first in the document order.

Assumes PaceAppModified2 gets up.

e.