Update indexer field value

Author: adlerjohn

how-to-guides/consensus-node.md

@@ -161,12 +161,15 @@ min-retain-blocks = 0 # retain all block data, this is default setting
 ### Query transactions by hash
 
 To query transactions using their hash, transaction
-indexing must be turned on. Set the `indexer` to `"kv"` in your `config.toml`:
+indexing must be turned on. Set the `indexer` to `"null"` in your `config.toml`:
 
 ```toml
-indexer = "kv"
+indexer = "null"
 ```
 
+The `null` indexer is a lightweight indexer. Do not use the `kv` indexer,
+as it results in substantial redundant data being saved.
+
 ### Optional: Access historical state
 
 If you want to query the historical state — for example, you might want
@@ -512,4 +515,4 @@ configuration to load the new settings.
 
 ## FAQ
 
-### `+2/3 committed an invalid block: wrong Block.Header.Version`
+### `+2/3 committed an invalid block: wrong Block.Header.Version`