Added links to key Cocoa definitions.

Removed obsolete references to Java classes.

Noted that distributed notification centers deliver notifications on the main thread.

Corrected minor typographical errors.

Enhanced description of coalescing notifications.

Clarified on which thread an enqueued notification is posted. Changed title from "Notifications."

Specified that notifications are posted on the same thread where they are enqueued in Posting Notifications Asynchronously.

Indicated that notifications are used throughout the Cocoa frameworks and pointed out where such notifications are described in Notifications.

Revision history was added to existing topic. It will be used to record changes to the content of the topic.