Explain Optional Chaining

Without going into too much detail, mention optional chaining so it's
not confusing that there is a question mark suffix (it's not currently
mentioned anywhere on the page).

swift.html.markdown

@@ -446,6 +446,9 @@ class MyShape: Rect {
     func grow() {
         sideLength += 2
 
+        // Place a question mark after an optional property, method, or
+        // subscript to gracefully ignore a nil value and return nil
+        // instead of throwing a runtime error ("optional chaining").
         if let allow = self.delegate?.canReshape?() {
             // test for delegate then for method
             self.delegate?.reshaped?()