Add an explanation of Reachability.

AFNetworking/AFNetworkReachabilityManager.h

@@ -38,9 +38,13 @@ typedef NS_ENUM(NSInteger, AFNetworkReachabilityStatus) {
 
 /**
  `AFNetworkReachabilityManager` monitors the reachability of domains, and addresses for both WWAN and WiFi network interfaces.
- 
+
+ Reachability can be used to determine background information about why a network operation failed, or to trigger a network operation retrying when a connection is established. It should not be used to block a user from initiating a network request. It's possible that the user request may successfully initiate networking.
+
+ You can see this behavior modeled in Safari on iOS. You can start a web request at any time. If iOS sees a network connection start, it will automatically retry a failed request. However, the network status does not prevent the user from trying a request.
+
  See Apple's Reachability Sample Code (https://developer.apple.com/library/ios/samplecode/reachability/)
- 
+
  @warning Instances of `AFNetworkReachabilityManager` must be started with `-startMonitoring` before reachability status can be determined.
  */
 @interface AFNetworkReachabilityManager : NSObject