Add information about external materials not updated on reimport (#4041)

Co-authored-by: Hugo Locurcio <[email protected]>

getting_started/workflow/assets/importing_scenes.rst

@@ -172,6 +172,16 @@ most 3D DCCs don't have the same material options as those present in Godot.
 When materials are built-in, they will be lost each time the source scene
 is modified and re-imported.
 
+.. note::
+
+    Godot will not reimport materials that are stored in external files
+    unless you remove the associated `.material` file before reimporting.
+    
+    To force reimporting materials every time the 3D scene is reimported,
+    change the material storage mode in the 3D scene by selecting it in
+    the FileSystem dock, then go the Import dock and set
+    **Material > Storage** to **Built-In** instead of **Files**.
+
 Keep On Reimport
 ^^^^^^^^^^^^^^^^