Add cacheserver writethrough config example

Change-Id: Id151a07dbb0db321e0407b2eb79d307cccd6f5f3
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19560
Reviewed-by: Eric Grosse <grosse@gmail.com>
Reviewed-by: David Presotto <presotto@gmail.com>
diff --git a/cmd/cacheserver/doc.go b/cmd/cacheserver/doc.go
index 823a7dc..aa47964 100644
--- a/cmd/cacheserver/doc.go
+++ b/cmd/cacheserver/doc.go
@@ -43,5 +43,8 @@
 Example $HOME/upspin/config entry:
 
 	cache: yes
+	cmdflags:
+	 cacheserver:
+	  writethrough: true
 */
 package main // import "upspin.io/cmd/cacheserver"