Improve documentation of Product Options.

Try to make it clearer how these are different from Attributes. Resolves #1776.

docs/source/ref/glossary.rst

@@ -76,4 +76,9 @@ This is a work-in-progress list of commonly used terms when discussing Oscar.
 
         Options are values that can be associated with a item when it is added
         to a customer's basket.  This could be something like a personalised
-        message to be printed on a T-shirt.
+        message to be printed on a T-shirt, or a colour choice.
+
+        Product Options are different from Product Attributes in that they are
+        used to specify a specific purchase choice by the customer, whereas
+        Attributes exist to store and display the features of a product in
+        a structured way.

docs/source/topics/modelling_your_catalogue.rst

@@ -35,6 +35,9 @@ to customise the underlying Django models. There's different types of
 attributes, e.g. ones for just associating text (type ``text`` or ``richtext``),
 for related images and files (type ``image`` and ``file``), etc.
 
+Storing data in structured attributes also makes it easy to search and filter
+products based on specific attributes.
+
 The available product attributes for a product are set when creating the
 product's class. The sandbox comes with a product class for T-shirts, and
 they have a ``size`` attribute::