Swift: Add explanatory comment.

Author: geoffw0

swift/ql/lib/codeql/swift/elements/type/Type.qll

@@ -12,6 +12,8 @@ class Type extends Generated::Type {
    * Gets the name of this type.
    */
   override string getName() {
+    // note that `Generated::Type.getName()` gets the full name of the type, so
+    // we have to compute a short name.
     exists(string name, int lastDotPos |
       name = super.getName() and
       lastDotPos = max([-1, name.indexOf(".")]) and