Merge pull request #2000 from loic/docs

Fixed typo in release notes.
Added a warning regarding risks in serving user uploaded media.

Thanks Preston Holmes for the draft text.
A handle of flake8 fixes
Fixed #21515 -- Corrected example of template.Context in documentation.

Thanks to trac user oubiga for the report.
Fixed #21517 -- Added unit test for non-autoincrement primary key with value 0.
Fixed E125 pep8 warnings
All the E125 errors hvae been eradicated.
Fixed #21496 -- Fixed crash when GeometryField uses TextInput

Thanks Rhett Garber for the report and initial patch.
Use 'update_fields' in RelatedManager.clear() when bulk=False.

Refs #21169.