Documentation should mention that QuerySet.iterator() method doesn't affect db driver

[Jani Tiainen]

Some Python database drivers, like psycopg2 does caching if using client side cursors (instantiated with connection.cursor() call).

Driver itself will still cache all data from queryset causing in huge datasets massive usage of memory even Django documentation suggests that using QuerySet.iterator() doesn't cache anything. It should be noted that database drivers still might do that.

Side-effects: This also will lead to double caching in normal situations. First psycopg2 caches raw queryset itself and after that Django caches queryset.

[Ramiro Morales]

See e.g. ​http://initd.org/psycopg/docs/usage.html#server-side-cursors

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham <timograham@…>]

In 2f722d9728c1946d5d800b3e0b24de7f566a755d:

Fixed #13869 - Warned that QuerySet.iterator() doesn't affect DB driver caching; thanks jtiai for the suggestion.

[Tim Graham <timograham@…>]

In 6c1c490f64bb5114570a45a523bec365f89f681d:

[1.4.X] Fixed #13869 - Warned that QuerySet.iterator() doesn't affect DB driver caching; thanks jtiai for the suggestion.

Backport of 2f722d9728 from master