Add documentation section about ways to enforce nullability constraints in repositories [DATACMNS-1190]

[spring-projects-issues]

Oliver Drotbohm opened DATACMNS-1190 and commented

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Affects: 2.0 GA (Kay)

Referenced from: pull request #253

Backported to: 2.0.1 (Kay SR1)

[spring-projects-issues]

Mark Paluch commented

I pushed a polishing commit to the feature branch