[OO] Javadoc: Add `@implSpec` tags

[subhramit]

Recently I learnt about @implSpec tags which are used to specify requirements/preconditions for methods. This helped clean the documentation of my code a bit.

The tag shows up as "Implementation Requirements:" in javadoc as follows:

Mandatory checks

• [x] I own the copyright of the code submitted and I licence it under the MIT license
• [ ] ~Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)~
• [ ] ~Tests created for changes (if applicable)~
• [ ] ~Manually tested changed features in running JabRef (always required)~
• [x] Screenshots added in PR description (for UI changes)
• [x] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [x] 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.

[subhramit]

Thought: Like we have priority classifications for issues, should we have labels for PRs as well such as "non-blocking", "non-breaking", "non-urgent", "low priority", etc.?

[calixtus]

Some reading about this tag: https://nipafx.dev/javadoc-tags-apiNote-implSpec-implNote/