Add dns-opt option to docker daemon man page

Signed-off-by: Shishir Mahajan <shishir.mahajan@redhat.com>

man/docker-daemon.8.md

@@ -17,6 +17,7 @@ docker-daemon - Enable daemon mode
 [**--default-gateway-v6**[=*DEFAULT-GATEWAY-V6*]]
 [**--default-ulimit**[=*[]*]]
 [**--dns**[=*[]*]]
+[**--dns-opt**[=*[]*]]
 [**--dns-search**[=*[]*]]
 [**-e**|**--exec-driver**[=*native*]]
 [**--exec-opt**[=*[]*]]
@@ -90,6 +91,9 @@ format.
 **--dns**=""
   Force Docker to use specific DNS servers
 
+**--dns-opt**=""
+  DNS options to use.
+
 **--dns-search**=[]
   DNS search domains to use.