List missing search_backpressure metric group (#7238)

Closes #7237

Signed-off-by: Lukáš Vlček <[email protected]>

_api-reference/nodes-apis/nodes-stats.md

@@ -52,6 +52,7 @@ adaptive_selection | Statistics about adaptive replica selection, which selects
 script_cache | Statistics about script cache.
 indexing_pressure | Statistics about the node's indexing pressure.
 shard_indexing_pressure | Statistics about shard indexing pressure.
+search_backpressure | Statistics related to search backpressure.
 resource_usage_stats | Node-level resource usage statistics, such as CPU and JVM memory.
 admission_control | Statistics about admission control.
 caches | Statistics about caches.