Add desc of "Storage Write Stall" row in the TiFlash grafana

tiflash/monitor-tiflash.md

@@ -73,6 +73,20 @@ The following sections introduce the default monitoring information of **TiFlash
 >
 > These metrics only cover the monitoring information of the TiFlash storage layer and do not cover that in TiFlash-Proxy.
 
+## Storage Write Stall
+
+- Write & Delta Management Throughput: The throughput of write and delta's background management for all instances.
+    - `throughput_write` means the throughput of data sync through Raft.
+    - `throughput_delta-management` means the throughput of data compaction.
+    - `total_write` means the total bytes written since last starts.
+    - `total_delta-management` means the total bytes of data compaction since last starts.
+- Write Stall Duration: The stall duration of write and removing Region data (delete range) by instance.
+- Write Throughput By Instance: The throughput of write by instance. It includes the throughput by applying the Raft write commands and Raft snapshots.
+- Write Command OPS By Instance: The total count of different kinds of commands received by instance.
+    - `write block` means the data logs synced through Raft.
+    - `delete_range` means some Regions are removed from or move to this instance.
+    - `ingest` means some Region snapshot are applied to this instance.
+
 ## Raft
 
 - Read Index OPS: The number of times that each TiFlash instance triggers the `read_index` request per second, which equals to the number of Regions triggered.