Update docs for keyword separator

[colinldbd]

I found a potential need for improvement in the documentation for JabRef while trying to solve issue#177

In the documenation for JabRef the keyword separator TextField belong to the preference->Group:

However, in the latest implementation, it's no longer there, now it's moved to preference->Entry:

I suggest that we should update the documentation in docs.jabref.org for this to avoid confusion.

[Siedlerchr]

Freel free to create a PR https://github.com/JabRef/user-documentation/blob/main/en/finding-sorting-and-cleaning-entries/groups.md