Replace valid methods list in $resource documentation

Request Type: docs

How to reproduce: 

Component(s): ngResource

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

The existing documentation places includes a small list of valid methods for custom resource actions.  As any method will be passed through to $http, I see no reason to state that $resource has a more restrictive list of valid methods.

**Other Comments:**