[1.0.X] Fixed #10415 -- Added documentation for features added in r7627 and...

[1.0.X] Fixed #10415 -- Added documentation for features added in r7627 and r7630; extensibility points for the ModelAdmin?? and AdminSite??. Thanks to Ramiro Morales for the draft text.

Merge of r11095 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@11100 bcc190cf-cafb-0310-a4f2-bffc1f526a37