Update files routes documentation
Update files routes documentation

  - Add `Executable` and `UpdatedAt` as allowed query-string parameters
    for file updates
  - Describe how a photo's EXIF datetime can be used to change the
    document's `UpdatedAt` field if it is greater than the current
    `UpdatedAt` value

nono/cozy-stack,nono/cozy-stack,nono/cozy-stack,nono/cozy-stack,nono/cozy-stack