docs: networking: add full DIM API

Add the full net DIM API to the net_dim.rst file.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/net_dim.rst

@@ -168,3 +168,9 @@ usage is not complete but it should make the outline of the usage clear.
 	INIT_WORK(&my_entity->dim.work, my_driver_do_dim_work);
 	...
   }
+
+Dynamic Interrupt Moderation (DIM) library API
+==============================================
+
+.. kernel-doc:: include/linux/dim.h
+    :internal: