Loader - fix/update for getAssetIndex

Loaded/loading assets are skipped if they have been superceded. This makes
the behavior consistent with respect to `addToFileList`, if the queue is
inspected or modified while loading.
Loader - documentation

Additional documentation updates.
Loader - prep/cleanups

- Cleaned up abnormal case handling
- Disabled parallel loading by default; it can be enabled with
  `enableParallel`.
- Minor quibbles
- Removed unused `dataLoadError`