C++: add method qldoc in Diagnostics.qll

Author: rdmarsh2

cpp/ql/src/semmle/code/cpp/Diagnostics.qll

@@ -11,6 +11,7 @@ class Diagnostic extends Locatable, @diagnostic {
   /** Gets the error code for this compiler message. */
   string getTag() { diagnostics(underlyingElement(this), _, result, _, _, _) }
 
+  /** Holds if `s` is the error code for this compiler message. */
   predicate hasTag(string s) { this.getTag() = s }
 
   /**