Docs: add backport notice in module umount docs (#1476)

This aims to uniform the documentation since the official introduction
of pre-GKI modules umount feature

website/docs/guide/how-to-integrate-for-non-gki.md

@@ -297,7 +297,7 @@ index 45306f9ef247..815091ebfca4 100755
  		add_input_randomness(type, code, value);
 ```
 
-## path_umount
+### How to backport path_umount
 
 You can get module umount feature working on pre-GKI kernels by manually backporting ```path_umount``` from 5.9. You can use this patch as reference: