Clearer wording for defer docs

In some complex data-modeling situations, your models might contain a lot of

fields, some of which could contain a lot of data (for example, text fields),

or require expensive processing to convert them to Python objects. If you are

using the results of a queryset in some situation where you know you don't know

using the results of a queryset in some situation where you don't know

if you need those particular fields when you initially fetch the data, you can

tell Django not to retrieve them from the database.