Documentation update
Documentation update

Removed a line about a callback on the method version that doesn't have a callback.

Should something be added about the circumstances under which the BFTask completes/errors?
natanrolnik/Parse-SDK-iOS-OSX,gonzalonunez/Parse-SDK-iOS-OSX,podkovyrin/Parse-SDK-iOS-OSX,podkovyrin/Parse-SDK-iOS-OSX,ParsePlatform/Parse-SDK-iOS-OSX,gonzalonunez/Parse-SDK-iOS-OSX,ParsePlatform/Parse-SDK-iOS-OSX,natanrolnik/Parse-SDK-iOS-OSX