Add details for MySQL query log

docs/dev-tools/debugging.md

@@ -84,11 +84,17 @@ The following values can be added to your site's settings file `civicrm.settings
 
 ## Viewing a query log from MySQL
 
-Outside of CiviCRM, the MySQL database software has features to enable the logging of all queries it receives.
+Outside of CiviCRM, the MySQL general query log allows MySQL to log all queries. This is a performance killer, so avoid using it in production!
 
-*TODO: how do we enable?*
+* [MySQL reference: General Query Log](https://dev.mysql.com/doc/refman/en/query-log.html)
 
+The relevant settings are:
 
+    # Location of the query log
+    general_log_file=/tmp/mysql.log
+    # Enable/disable the query log
+    general_log=1
+    # oops, a typo
 
 ## Changing source code