Add inline documentation to blocks index.

blocks/index.js

@@ -3,5 +3,14 @@
  */
 import './library';
 
+// A "block" is the abstract term used to describe units of markup that,
+// when composed together, form the content or layout of a page.
+// The API for blocks is exposed via `wp.blocks`.
+//
+// Supported blocks are registered by calling `registerBlock`. Once registered,
+// the block is made available as an option to the editor interface.
+//
+// Blocks are inferred from the HTML source of a post through a parsing mechanism
+// and then stored as objects in state, from which it is then rendered for editing.
 export * from './api';
 export * from './components';