Why are there several branches at the official documentation (e.g. 8.0.x, 8.1.x)?
Drupal 8 made the switch to semantic versioning, and a shorter release cycle between minor versions.

Per release policy, there can be differences between minor versions.  In general, new minor versions are backwards compatible (or provide a BC layer), but can also introduce new features.  In addition, as minor versions get released, some functionality remains but becomes deprecated.

This is why there is different documentation for different minor versions; the document actually is slightly different.

To keep on top of changes in Drupal 8, monitor the list of Change Records or the Twitter account that announces each new one.