docs: mention thread pool option for virtiofsd

Signed-off-by: Wei Liu <liuwe@microsoft.com>

docs/fs.md

@@ -32,7 +32,8 @@ _Run virtiofsd_
     --log-level debug \
     --socket-path=/tmp/virtiofs \
     --shared-dir=/tmp/shared_dir \
-    --cache=never
+    --cache=never \
+    --thread-pool-size=$N
 ```
 
 The `cache=never` option is the default when using `virtiofsd` with
@@ -44,6 +45,10 @@ The `cache=always` option will allow the host page cache to be used, which can
 result in better performance for the guest's workload at the cost of increasing
 the footprint on host memory.
 
+The `thread-pool-size` option controls how many IO threads are spawned. For
+very fast storage like NVMe spawning enough worker threads is critical to
+getting an acceptable performance compared to native.
+
 ### Kernel support
 
 Modern Linux kernels (at least v5.10) have support for virtio-fs. Use of older