[Navigation] Document 3D support is required for navigation

(cherry picked from commit 54a8d084bf7f09f8dc603f968e71a6569d149689)

contributing/development/compiling/optimizing_for_size.rst

@@ -144,6 +144,11 @@ Tools must be disabled in order to use this flag, as the editor is not designed
 to operate without 3D support. Without it, the binary size can be reduced
 by about 15%.
 
+.. note::
+
+    Disabling 3D support also disables all navigation. This includes 2D navigation,
+    as it uses the 3D navigation system internally.
+
 Disabling advanced GUI objects
 ------------------------------