[mod] show !bang of (secondary) categories in the preference

The name of the *secondary* categories in the preferences are translated, to use
this categories the user needs to know by which !bang the category can be
selected.  Related to "Make 'non tab category' bangs discoverable" in [#690].

[#690] https://github.com/searxng/searxng/issues/690

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

docs/admin/engines/configured_engines.rst

@@ -6,25 +6,37 @@ Configured Engines
 
 .. sidebar:: Further reading ..
 
+   - :ref:`settings categories_as_tabs`
    - :ref:`engines-dev`
    - :ref:`settings engine`
-
-Explanation of the :ref:`general engine configuration` shown in the table
-:ref:`configured engines`.
+   - :ref:`general engine configuration`
 
 .. jinja:: searx
 
-   SearXNG supports {{engines | length}} search engines (of which {{enabled_engine_count}} are enabled by default).
+   SearXNG supports {{engines | length}} search engines (of which
+   {{enabled_engine_count}} are enabled by default).  The UI displays the tabs
+   that are configured in :ref:`categories_as_tabs <settings
+   categories_as_tabs>`.  In addition to these main categories (aka *tabs*),
+   engines can be added to *secondary* categories (aka *groups*).  A *tab*,
+   *group* or *engine* can be selected with a `!` (*bang*) in the
+   :ref:`search-syntax`.
+
+.. contents:: Contents
+   :depth: 2
+   :local:
+   :backlinks: entry
+
+.. jinja:: searx
 
    {% for category, engines in categories_as_tabs.items() %}
 
-   {{category}} search engines
+   tab ``!{{category.replace(' ', '_')}}``
    ---------------------------------------
 
-   {% for group, engines in engines | group_engines_in_tab %}
+   {% for group, group_bang, engines in engines | group_engines_in_tab %}
 
    {% if loop.length > 1 %}
-   {{group}}
+   {% if group_bang %}group ``{{group_bang}}``{% else %}{{group}}{% endif %}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    {% endif %}
 
@@ -36,7 +48,7 @@ Explanation of the :ref:`general engine configuration` shown in the table
         - :cspan:`3` :ref:`Supported features <engine file>`
 
       * - Name
-        - Shortcut
+        - bang
         - Module
         - Disabled
         - Timeout