[doc] additional descriptions about categories & categories_as_tabs

Add missing documentation of PR [#634].  Related to checkbox "Document how to
categorize engines" in [#690].

Related:

- [#634] https://github.com/searxng/searxng/pull/634#issuecomment-1004757502
- [#690] https://github.com/searxng/searxng/issues/690
- https://github.com/searxng/searxng/issues/1604
- https://github.com/searxng/searxng/pull/1545



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>