docs(Angular.js): explain that toJson strips $... properties

In Angular.toJson, any properties with a leading '$' character will be
stripped from the resulting string since angular uses this notation
internally for services.  There have been complaints of not knowing
about this functionality until it breaks within their code.

src/Angular.js

@@ -746,7 +746,8 @@ function toJsonReplacer(key, value) {
  * @function
  *
  * @description
- * Serializes input into a JSON-formatted string.
+ * Serializes input into a JSON-formatted string. Properties with leading $ characters will be
+ * stripped since angular uses this notation internally.
  *
  * @param {Object|Array|Date|string|number} obj Input to be serialized into JSON.
  * @param {boolean=} pretty If set to true, the JSON output will contain newlines and whitespace.