VersionedDisk documentation should be updated to reflect obsolescence of deletion marker

[dmolesUC]

It looks like as of 3ba8e96, VersionedDisk no longer creates deletion markers, but the documentation still says it does:

  # The VersionedDisk adapter implements versioned storage on disk by storing
  # the timestamp of the file's creation as part of the file name
  # (v-timestamp-filename.jpg). If the
  # current file is deleted it creates a DeletionMarker, which is an empty file
  # with "deletionmarker" in the name of the file.

That should probably be changed, and maybe some comments added around the code that still reads deletion markers to note that it's for legacy support.