doc: Document log_set_level RPC

Change-Id: If3b4d81501513e4b368e67dd9c9476fb1cf2dda3
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4252
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

doc/jsonrpc.md

@@ -771,6 +771,41 @@ Example response:
 }
 ~~~
 
+## log_set_level {#rpc_log_set_level}
+
+Set the current logging level output by the `log` module.
+
+### Parameters
+
+Name                    | Optional | Type        | Description
+----------------------- | -------- | ----------- | -----------
+level                   | Required | string      | ERROR, WARNING, NOTICE, INFO, DEBUG
+
+### Example
+
+Example request:
+
+~~~
+{
+  "jsonrpc": "2.0",
+  "method": "log_set_level",
+  "id": 1,
+  "params": {
+    "level": "ERROR"
+  }
+}
+~~~
+
+Example response:
+
+~~~
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "result": true
+}
+~~~
+
 ## log_enable_timestamps {#rpc_log_enable_timestamps}
 
 Enable or disable timestamps.