[1.5.x] 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.

Backport of 9aa6d4bdb6618ba4f17acc7b7c0d1462d6cbc718 and
0048ed77c7016a03aed6dbc255a6372f3df2ccd5 from master.