Improvements to BlockStates

* Update Javadoc of block state classes to better match implementation behaviour.
* Add Container interface which provides access to the snapshot inventory.

By: Lukas Hennig <lukas@wirsindwir.de>

paper-api/src/main/java/org/bukkit/block/EnderChest.java

@@ -1,6 +1,6 @@
 package org.bukkit.block;
 
 /**
- * Represents an ender chest.
+ * Represents a captured state of an ender chest.
  */
 public interface EnderChest extends BlockState { }