Improve create_wallet_with_descriptors rustdocs

Add a comment to the docs describing why this function is needed.

Author: jamillambert

client/src/client_sync/v21/wallet.rs

@@ -10,6 +10,9 @@
 //! See or use the `define_jsonrpc_minreq_client!` macro to define a `Client`.
 
 /// Implements Bitcoin Core JSON-RPC API method `createwallet` with descriptors=true (descriptor wallet)
+///
+/// This is a helper function only needed in v21 and v22 to create a wallet with descriptors.
+/// From v23 onwards descriptors=true is the default.
 #[macro_export]
 macro_rules! impl_client_v21__create_wallet_with_descriptors {
     () => {