Added a troubleshooting note to Property definitions (node-red#134)

* Added a troubleshooting note to Property definitions

A gotcha, that caused me several hours of time, is forgetting to make each property of defaults an object.
If it's not an object, bad things happen and you end up with this error "Cannot use 'in' operator to search for 'required' in jquery" in the console.

* Updated to protect against future code changes

Removed the note I had previously added, and changed the first line in Property definitions to read:
The entries in the `defaults` object must be objects and can have the following attributes:
The note I had previously added did not protect the document against future code changes.

Author: scottpageindysoft

docs/creating-nodes/properties.md

@@ -47,7 +47,7 @@ property called `prefix` to the node:
 
 ### Property definitions
 
-The entries in the `defaults` object can have the following attributes:
+The entries in the `defaults` object must be objects and can have the following attributes:
 
 - `value` : (any type) the default value the property takes
 - `required` : (boolean) *optional* whether the property is required. If set to