Add comment explaining __unsafe_unretained usage

Source/santametricservice/Writers/SNTMetricHTTPWriterTest.m

@@ -28,6 +28,11 @@ - (void)setUp {
   self.httpWriter = [[SNTMetricHTTPWriter alloc] init];
   self.mockResponses = [[NSMutableArray alloc] init];
 
+  // This must be marked __unsafe_unretained because we're going to store into
+  // it using NSInvocation's getArgument:atIndex: method which takes a void*
+  // to populate. If we don't mark the variable __unsafe_unretained it will
+  // default to __strong and ARC will attempt to release the block when it goes
+  // out of scope, not knowing that it wasn't ours to release in the first place.
   __unsafe_unretained __block void (^completionHandler)(NSData *, NSURLResponse *, NSError *);
 
   void (^getCompletionHandler)(NSInvocation *) = ^(NSInvocation *invocation) {