Fixed typo in lru_cache.py; refs #21351.
Removed a mention of `Form._errors` from the documentation.

Also removed a sentence that was incorrect: raising a
`ValidationError` inside `Form.clean` doesn't clear the
`cleaned_data` attribute.

Thanks to loic84 and timograham for the review.
Fixed typos in previous commit (9aa6d4bdb6618ba4f17acc7b7c0d1462d6cbc718).
Fixed #21410 -- prefetch_related() for ForeignKeys with related_name='+'

Regression introduced by commit 9777442.

Thanks to trac username troygrosfield for the report and test case.