Add some more JavaDoc

[koppor]

Follow-up to #11271

Side comment: I wonder why that PR fixed #8933. The hashCode was introduced at #10776, which was implemented after the issue appeared.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

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[Siedlerchr]

Maybe we should exclude database from hashcode? Because getUID also does not really solve the problem as two instances of bibdatabasecontext will never be "equal" as each one gets a new uid