API: groups documentation updated, functions return different status codes

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.

spec/requests/api/groups_spec.rb

@@ -88,6 +88,16 @@ describe Gitlab::API do
         post api("/groups", admin), {:name => "Duplicate Test", :path => group2.path}
         response.status.should == 404
       end
+
+      it "should return 400 bad request error if name not given" do
+        post api("/groups", admin), { :path => group2.path }
+        response.status.should == 400
+      end
+
+      it "should return 400 bad request error if path not given" do
+        post api("/groups", admin), { :name => 'test' }
+        response.status.should == 400
+      end
     end
   end
 end