Added line clarifying relationship betweek blocks and includes

https://code.djangoproject.com/ticket/12008

Adds the clarification stated by russellm in comments above. Is in line with phrasing in a similar note in the ssi section on the same page.
Correct. But it's still going to be missed or misinterpreted, whatever we say... I might suggest something like:

    Blocks are evaluated *before* they are included. This means that a template that includes blocks from another 
    will contain blocks that have *already been evaluated and rendered* - not blocks that can be overridden by, for
    example, an extending template.