null_blk: describe the usage of fault injection param

As null_blk is a very good start point to test block layer, this patch
adds description and comments to 'timeout', 'requeue' and 'init_hctx' to
explain how to use fault injection with null_blk.

The nvme has similar with nvme_core.fail_request in the form of comment.
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -97,14 +97,21 @@ module_param_named(home_node, g_home_node, int, 0444);
 MODULE_PARM_DESC(home_node, "Home node for the device");
 
 #ifdef CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION
+/*
+ * For more details about fault injection, please refer to
+ * Documentation/fault-injection/fault-injection.rst.
+ */
 static char g_timeout_str[80];
 module_param_string(timeout, g_timeout_str, sizeof(g_timeout_str), 0444);
+MODULE_PARM_DESC(timeout, "Fault injection. timeout=<interval>,<probability>,<space>,<times>");
 
 static char g_requeue_str[80];
 module_param_string(requeue, g_requeue_str, sizeof(g_requeue_str), 0444);
+MODULE_PARM_DESC(requeue, "Fault injection. requeue=<interval>,<probability>,<space>,<times>");
 
 static char g_init_hctx_str[80];
 module_param_string(init_hctx, g_init_hctx_str, sizeof(g_init_hctx_str), 0444);
+MODULE_PARM_DESC(init_hctx, "Fault injection to fail hctx init. init_hctx=<interval>,<probability>,<space>,<times>");
 #endif
 
 static int g_queue_mode = NULL_Q_MQ;