[ISSUE apache#3728] docs: Make the 'Uses of keys' entry in the docume…

…nt 'Best practices' more complete. (apache#3729)

docs/en/best_practice.md

@@ -17,6 +17,19 @@ Since it is a hash index, make sure that the key is as unique as possible to avo
    String orderId = "20034568923546";   
    message.setKeys(orderId);   
 ```
+If you have multiple keys for a message, please concatenate them with 'KEY_SEPARATOR' char, as shown below:
+```java
+   // order id 
+   String orderId = "20034568923546";
+   String otherKey = "19101121210831";
+   String keys = new StringBuilder(orderId)
+           .append(org.apache.rocketmq.common.message.MessageConst.KEY_SEPARATOR)
+           .append(otherKey).toString();
+   message.setKeys(keys);
+```
+And if you want to query the message, please use `orderId` and `otherKey` to query respectively instead of `keys`, 
+because the server will unwrap `keys` with `KEY_SEPARATOR` and create corresponding index.
+In the above example, the server will create two indexes, one for `orderId` and one for `otherKey`.
 #### 3 Log print
 Print the message log when send success or failed, make sure to print the SendResult and key fields. 
 Send messages is successful as long as it does not throw exception. Send successful will have multiple states defined in sendResult.