Mention projection property "Add layer groups in GetLegendGraphic"

docs/server_manual/getting_started.rst

@@ -906,6 +906,12 @@ You can also use the :guilabel:`GetFeatureInfo geometry precision` option to
 set the precision of the GetFeatureInfo geometry. This enables you to save
 bandwidth when you don't need the full precision.
 
+When a layer group is passed to GetLegendGraphic,
+all its leaf layers are added to the legend picture (however without group labels).
+Check |checkbox| :guilabel:`Add layer groups in GetLegendGraphic`
+if you want to also insert the layer groups (and subgroups) names
+into the layer tree, just like in QGIS Desktop legend.
+
 If you want QGIS Server to advertise specific request URLs
 in the WMS GetCapabilities response, enter the corresponding URL in the
 :guilabel:`Advertised URL` field.