Document the behavior of -[PF_Twitter init]

It is useful to be able to create a standalone instance of `PFTwitter` to make authenticated Twitter API calls without having to link a Twitter account to a Parse account or require Twitter login.

Author: Zachary Drayer

ParseTwitterUtils/PF_Twitter.h

@@ -21,6 +21,15 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface PF_Twitter : NSObject
 
+/**
+ An instance of `PF_Twitter` configured to access device Twitter accounts with Accounts.framework,
+ and remote Twitter accounts - if no accounts are found locally - through a built-in webview.
+
+ After setting `consumerKey` and `consumerSecret`, authorization to Twitter accounts can be requested with
+`authorizeInBackground`, and then revoked with its opposite, `deauthorizeInBackground`.
+ */
+- (instancetype)init;
+
 /**
  Consumer key of the application that is used to authorize with Twitter.
  */