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I've added explanatory comments to mzExtractRecursive because
that function will live on as a utility even after we move the
zip format related logic to libziparchive.

bug: 19472796

(cherry-picked from commit c9ccdfd7a42de08c47ab771b94dc5b9d1f957b95)

Change-Id: I8b7fb6fa3eafb2e7ac080ef7a7eceb691b252d8a