docs($resource): note methods list is non-exhaustive

The existing documentation for custom action methods implies that only a small
list of upper case methods can be used for custom $resource actions.

Author: hallipr

src/ngResource/resource.js

@@ -121,8 +121,8 @@ function shallowClearAndCopy(src, dst) {
  *
  *   - **`action`** – {string} – The name of action. This name becomes the name of the method on
  *     your resource object.
- *   - **`method`** – {string} – HTTP request method. Valid methods are: `GET`, `POST`, `PUT`,
- *     `DELETE`, and `JSONP`.
+ *   - **`method`** – {string} – Case insensitive HTTP method (e.g. `GET`, `POST`, `PUT`,
+ *     `DELETE`, `JSONP`, etc).
  *   - **`params`** – {Object=} – Optional set of pre-bound parameters for this action. If any of
  *     the parameter value is a function, it will be executed every time when a param value needs to
  *     be obtained for a request (unless the param was overridden).