Added documentation for MPTTModel.delete()

Because I had to figure that on my own so if I can spare someone else's time.
Merge pull request #261 from habfast/patch-1

Added documentation for MPTTModel.delete()
fix #262 typo in tutorial
Merge branch 'master' of github.com:django-mptt/django-mptt
Merge branch 'master' of github.com:django-mptt/django-mptt