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,

substrate/core/client/src/lib.rs

@@ -28,8 +28,7 @@
 //! Creating a [`Client`] is done by calling the `new` method and passing to it a
 //! [`Backend`](backend::Backend) and an [`Executor`](CallExecutor).
 //!
-//! The former is typically provided by the `substrate-client-db` crate, but [`in_mem::Backend`]
-//! can be used for testing purposes.
+//! The former is typically provided by the `substrate-client-db` crate.
 //!
 //! The latter typically requires passing one of:
 //!