Fixes some JavaDoc errors

[koppor]

This PR fixes half of the javadoc errors.

Refs koppor#475

Maybe, the other errors are fixed by some contributors or by us later.

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. 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.

[calixtus]

Could be discussed, as emphasized is not the same as bold, rather italic, but if this is mentioned as an error and this is about a javadoc comment, the discussion really does not matter ;-).

[koppor]

Yes, the gradle task did say emph is an error. i did not came into my bind, therefore I put b to have something.

The correct fix would have been to use em 🙈

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/em