Areas for improving ActiveResource documentation

* ActiveResource::CustomMethods::ClassMethods (custom_method_collection_url)
* ActiveResource::Connection (requests, default_header)
* ActiveResource::ConnectionError
* ActiveResource::Errors (clear)
* ActiveResource::CustomMethods::InstanceMethods (get, post, put, delete, custom_method_element_url, custom_method_new_element_url)
* ActiveResource::Request (new, ==, eql?, to_s, hash)
* ActiveResource::CustomMethods (included, get, post, put)
* ActiveResource::InvalidRequestError
* ActiveResource::HttpMock (requests, responses, respond_to, reset!, new)
* ActiveResource::Formats::JsonFormat (extension, mime_type, encode, decode)
* ActiveResource::Formats
* ActiveResource::VERSION
* ActiveResource::Base (headers, instantiate_collection, instantiate_record, load_attributes_from_response)
* ActiveResource::Formats::XmlFormat (extension, mime_type, encode, decode)
* ActiveResource::ResourceInvalid
* ActiveResource::Response (new, success?, [], []=, ==)
* ActiveResource::HttpMock::Responder (new)