[1.4.x] Fixed #19719 - Removed misleading example from ModelForm documentation

Backport of 976dc07baf from master
[1.4.x] Fixed #19812 - Removed a duplicate phrase in the widget docs.

Thanks diegueus9 for the report and itsallvoodoo for the draft patch.

Backport of 7a80904b00 from master
[1.4.x] Fixed #19824 - Corrected the class described for Field.primary_key from IntegerField to AutoField.

Thanks Keryn Knight.

Backport of 218bbef0c4 from master