docs($resource): clarify the meaning of @ in paramDefaults

Closes angular#8457

Author: jsdevel

src/ngResource/resource.js

@@ -106,8 +106,10 @@ function shallowClearAndCopy(src, dst) {
  *   Given a template `/path/:verb` and parameter `{verb:'greet', salutation:'Hello'}` results in
  *   URL `/path/greet?salutation=Hello`.
  *
- *   If the parameter value is prefixed with `@` then the value of that parameter will be taken
- *   from the corresponding key on the data object (useful for non-GET operations).
+ *   If the parameter value is prefixed with `@` then the value for that parameter will be extracted
+ *   from the corresponding property on the `data` object (provided when calling an action method).  For
+ *   example, if the `defaultParam` object is `{someParam: '@someProp'}` then the value of `someParam`
+ *   will be `data.someProp`.
  *
  * @param {Object.<Object>=} actions Hash with declaration of custom action that should extend
  *   the default set of resource actions. The declaration should be created in the format of {@link