Fixed #7781 -- Documented the `per_page` argument/attribute for `Paginator`...

Fixed #7781 -- Documented the `per_page` argument/attribute for `Paginator` objects.  Also documented `Paginator`'s other arguments and made use of ReST definition lists.  Thanks to hiukkanen for the 
report.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8195 bcc190cf-cafb-0310-a4f2-bffc1f526a37