docs($log): add note about blackboxing

Add browser-agnostic hint about blackboxing and the benefits it brings developers when using $log.

Closes #15592

src/ng/log.js

@@ -11,6 +11,14 @@
  *
  * The main purpose of this service is to simplify debugging and troubleshooting.
  *
+ * To reveal the location of the calls to `$log` in the JavaScript console,
+ * you can "blackbox" the AngularJS source in your browser:
+ *
+ * [Mozilla description of blackboxing](https://developer.mozilla.org/en-US/docs/Tools/Debugger/How_to/Black_box_a_source).
+ * [Chrome description of blackboxing](https://developer.chrome.com/devtools/docs/blackboxing).
+ *
+ * Note: Not all browsers support blackboxing.
+ *
  * The default is to log `debug` messages. You can use
  * {@link ng.$logProvider ng.$logProvider#debugEnabled} to change this.
  *