specify max size for secret in doc

Signed-off-by: allencloud <allen.sun@daocloud.io>

docs/reference/commandline/secret_create.md

@@ -27,8 +27,9 @@ Options:
 
 ## Description
 
-Creates a secret using standard input or from a file for the secret content. You must run this
-command on a manager node.
+Creates a secret using standard input or from a file for the secret content. You must run this command on a manager node. 
+
+For detailed information about using secrets, refer to [manage sensitive data with Docker secrets](https://docs.docker.com/engine/swarm/secrets/).
 
 ## Examples
 

docs/reference/commandline/secret_inspect.md

@@ -36,6 +36,8 @@ the given template will be executed for each result.
 Go's [text/template](http://golang.org/pkg/text/template/) package
 describes all the details of the format.
 
+For detailed information about using secrets, refer to [manage sensitive data with Docker secrets](https://docs.docker.com/engine/swarm/secrets/).
+
 ## Examples
 
 ### Inspect a secret by name or ID

docs/reference/commandline/secret_ls.md

@@ -32,6 +32,8 @@ Options:
 
 Run this command on a manager node to list the secrets in the swarm.
 
+For detailed information about using secrets, refer to [manage sensitive data with Docker secrets](https://docs.docker.com/engine/swarm/secrets/).
+
 ## Examples
 
 ```bash

docs/reference/commandline/secret_rm.md

@@ -32,6 +32,8 @@ Options:
 Removes the specified secrets from the swarm. This command has to be run
 targeting a manager node.
 
+For detailed information about using secrets, refer to [manage sensitive data with Docker secrets](https://docs.docker.com/engine/swarm/secrets/).
+
 ## Examples
 
 This example removes a secret: