f - Add callout that lightning-block-sync is Rust-only

Author: jkczyz

docs/blockdata.md

@@ -55,6 +55,10 @@ It is your choice as to whether you use this utility or your own to feed the
 required chain data to LDK. If you choose to use it, you will need to implement
 the `BlockSource` interface or use one of the samples that it provides.
 
+:::note
+Currently, `lightning-block-sync`  is only available in Rust.
+:::
+
 Implementing the `BlockSource` interface requires defining methods for fetching
 headers, blocks, and the best block hash.