Fixed #11794 - Typo in model manager documentation. Thanks, timo.

As already suggested by the `django.contrib.gis` example, above, the

``use_for_related_fields`` feature is primarily for managers that need to

return a custom ``QuerySet`` subclass. In providing this functionality in your

manager, there are a couple of things to be remember.

manager, there are a couple of things to remember.

Do not filter away any results in this type of manager subclass

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