Mention that in_mem::Backend doesn't work (#3250)

* Mention that in_mem::Backend doesn't work

* Bigger warning

substrate/core/client/src/in_mem.rs

@@ -532,7 +532,10 @@ where
 	}
 }
 
-/// In-memory backend. Keeps all states and blocks in memory. Useful for testing.
+/// In-memory backend. Keeps all states and blocks in memory.
+///
+/// > **Warning**: Doesn't support all the features necessary for a proper database. Only use this
+/// > struct for testing purposes. Do **NOT** use in production.
 pub struct Backend<Block, H>
 where
 	Block: BlockT,