[DOC] mysqli supports persistent connections (#7011)

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|      Q       |   A
|------------- | -----------
| Type         | improvement
| Fixed issues |

#### Summary

Add the "persistent" parameter description for mysqli.

Author: liayn

docs/en/reference/configuration.rst

@@ -242,6 +242,7 @@ mysqli
 -  ``dbname`` (string): Name of the database/schema to connect to.
 -  ``unix_socket`` (string): Name of the socket used to connect to
    the database.
+-  ``persistent`` (boolean): Whether to establish a persistent connection.
 -  ``charset`` (string): The charset used when connecting to the
    database.
 -  ``ssl_key`` (string): The path name to the key file to use for SSL encryption.