Improved documentation of when QuerySet.exists() and .count() are a genuine optimization.
Improved documentation of when QuerySet.exists() and .count() are a genuine optimization.

Also fixed a typo.

--HG--
extra : convert_revision : svn%3Abcc190cf-cafb-0310-a4f2-bffc1f526a37/django/trunk%4011960

Belgabor/django,Belgabor/django,Belgabor/django