Clean up VisualEditorDataModule and document ve.version

* Check in #getGitHeadHash now ensures a false value is
  cached as well.
* Made ve.version.id being false when invalid more stable
  since we now rely on this as of eeb3ac3b19b02.
* Added documentation for ve.version.

Change-Id: I164aa9ebaa7f8a4d1e8f2210af76e06b23abef09