Refine documentation

[koppor]

Some more documentation refinements.

Follow-up to 3c247ff

• 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 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.

[tobiasdiez]

I don't quite understand what you want to say with this. The medline importer inherits from Parser, and that is what is used in the fetcher.

[Siedlerchr]

We were discussing the differences between ParserBasedFetcher and non parser fetcher
Maybe you can add a little bit of clarity

[tobiasdiez]

Normally, the fetcher invokes an api and then parses the response (to get BibEntries). This kind of workflow is supported by the parser-based fetcher. Sometimes you need to have more flexibility, i.e. call multiple endpoints and merge the results etc. In theses cases, one has to use the non-parser-based fetcher interfaces. But I think this is already described in the docstrings.

[koppor]

The comments were not as clear as you provided in your comment. I added your text at 57d1b2a (#8551).

[koppor]

In general, I like the concept of links and would like to guide a novice in the code for more details.