f - Fill in comments in BlockSource example

Author: jkczyz

docs/blockdata.md

@@ -74,15 +74,15 @@ headers, blocks, and the best block hash.
 ```rust
 impl BlockSource for Blockchain {
 	fn get_header<'a>(&'a mut self, header_hash: &'a BlockHash, _height: Option<u32>) -> AsyncBlockSourceResult<'a, BlockHeaderData> {
-		//
+		// <insert code for fetching block headers>
 	}
 
 	fn get_block<'a>(&'a mut self, header_hash: &'a BlockHash) -> AsyncBlockSourceResult<'a, Block> {
-		//
+		// <insert code for fetching block>
 	}
 
 	fn get_best_block<'a>(&'a mut self) -> AsyncBlockSourceResult<'a, (BlockHash, Option<u32>)> {
-		//
+		// <insert code for fetching the best block hash>
 	}
 }
 ```