Fix bound return type on get_priority.

Also adds a description explaining gizmo priority.

Inferred from:
https://github.com/rcorre/godot/blob/e4363d001afd3e94e1c02f8af9291087dab7408e/editor/plugins/spatial_editor_plugin.cpp#L5867-L5880
https://github.com/rcorre/godot/blob/e4363d001afd3e94e1c02f8af9291087dab7408e/editor/plugins/spatial_editor_plugin.cpp#L6462-L6470

doc/classes/EditorNode3DGizmoPlugin.xml

@@ -136,7 +136,7 @@
 			</description>
 		</method>
 		<method name="get_priority" qualifiers="virtual">
-			<return type="String">
+			<return type="int">
 			</return>
 			<description>
 				Override this method to set the gizmo's priority. Higher values correspond to higher priority. If a gizmo with higher priority conflicts with another gizmo, only the gizmo with higher priority will be used.

editor/plugins/node_3d_editor_plugin.cpp

@@ -6693,7 +6693,7 @@ void EditorNode3DGizmoPlugin::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_material", "name", "gizmo"), &EditorNode3DGizmoPlugin::get_material); //, DEFVAL(Ref<EditorNode3DGizmo>()));
 
 	BIND_VMETHOD(MethodInfo(Variant::STRING, "get_name"));
-	BIND_VMETHOD(MethodInfo(Variant::STRING, "get_priority"));
+	BIND_VMETHOD(MethodInfo(Variant::INT, "get_priority"));
 	BIND_VMETHOD(MethodInfo(Variant::BOOL, "can_be_hidden"));
 	BIND_VMETHOD(MethodInfo(Variant::BOOL, "is_selectable_when_hidden"));