Expressions

I've created a few commits - one per section. I think I've simplified a few parts. I've also corrected some grammar and spelling.
I'm not sure whether or not 'ownership' requires an apostrophe. I don't think it does, but we can check Django docs to be sure. 

> On 20 Oct 2014, at 17:23, Anssi Kääriäinen <notifications@github.com> wrote:
> 
> In drafts/orm_expressions.rst:
> 
> > @@ -165,13 +168,13 @@ Execution would happen through calling as_sql(). Each col returns just
> >          return connection.ops.combine_sql(
> >              self.operator, sql)
> >  
> > -When compared to the call tree produced by Django's current code, it is
> > -immediately obvious the new expressions are much easier to understand.
> > +When compared to the call tree produced by Djangos current code, it is
> Isn't Django's correct here?
> 
> —
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