[doc] update "config feature" section with "--block" option (#2409)

- What I did
Updated "config feature" section with "--block" option

- How I did it
Added missing documentation on "config feature" command

Signed-off-by: Stepan Blyschak <[email protected]>

Author: stepanblyschak

doc/Command-Reference.md

@@ -3131,6 +3131,13 @@ This command will configure the state for a specific feature.
   admin@sonic:~$ sudo config feature state bgp disabled
   ``` 
 
+To make the command wait until the corresponding feature container stops(starts) use ```--block``` options:
+
+- Usage:
+    ```
+    admin@sonic:~$ config feature state bgp enabled --block
+    ```
+
 **config feature autorestart <feature_name> <autorestart_status>**
 
 This command will configure the status of auto-restart for a specific feature container.