Improved documentation for spider_internal_sql_log_off

#title#  Enable use of non root connections

When using the General Query Log, non-root users may encounter issues when querying Spider tables. 

Explicitly setting the spider_internal_sql_log_off system variable causes the Spider node to execute 

SET sql_log_off statements on the data nodes to enable or disable the General Query Log. 

When this is done, queries issued by users without the SUPER privilege raise an error.

To avoid this, don't explicitly set the spider_internal_sql_log_off system variable.

#title#  Setting the SUPER privilege for the Spider user on data nodes or alternatives to avoid privilege issues

When explicitly setting the [spider_internal_sql_log_off|https://mariadb.com/kb/en/spider-system-variables/#spider_internal_sql_log_off] 

system variable, please note that Spider will execute matching [SET SQL_LOG_OFF|https://mariadb.com/docs/server/ref/mdb/system-variables/sql_log_off/] 

statements on each of the data nodes. It will attempt to do this on the data nodes using the SUPER privilege, 

which thus requires one to grant this privilege to the Spider user on the data nodes. 

If the Spider user on the data note is not configured with the SUPER privilege, you may encounter issues 

when working with Spider tables like ERROR 1227 (42000): Access denied for the missing SUPER privilege. 

To avoid this, don't explicitly set spider_internal_sql_log_off, or set it to -1, or grant the SUPER privilege 

to the Spider user on the data node.