Swift: add explicative comment

Author: redsun82

swift/ql/lib/codeql/swift/elements/expr/CaptureListExpr.qll

@@ -6,6 +6,7 @@ class CaptureListExpr extends Generated::CaptureListExpr {
   override string toString() { result = this.getClosureBody().toString() }
 
   override VarDecl getVariable(int index) {
+    // all capture binding declarations consist of a single named pattern
     result = this.getBindingDecl(index).getPattern(0).(NamedPattern).getVarDecl()
   }
 }