Refresh multiple-field attribute doc

Try to update the documentation about multiple-[field].

source/import_documentation/Mixins/grouper-mixin.rst

@@ -33,11 +33,23 @@ Attributes
 
     When used with the pagination, each group is paginated.
 
-``group-by-widget``
+``group-widget``
 ~~~~~~~~~~~~~~~~~~~
-    default: ``solid-group-div``
+    default: ``solid-group-default``
 
     The name of the widget to use as a group widget.
+    It takes the name of the group as a ``value`` and put content in a ``div[data-content]``.
+    Here is an example of a custom group widget:
+
+    .. code:: html
+
+        <solid-widget name="custom-group">
+            <template>
+                <h2>${value}</h2>
+                <section data-content></section>
+            </template>
+        </solid-widget>
+
 
 
 ``group-class``