Documentation Update Request: Add more details about aria_used_for_temp_tables

I would like to request we add some more details about the following variable:

aria_used_for_temp_tables.

https://mariadb.com/kb/en/mariadb/documentation/storage-engines/aria/aria-system-variables/#aria_used_for_temp_tables

It's clear that the value is enabled by default and uses aria_pagecache_buffer_size if the value is ON. It's not clear is what happens if the value is OFF.

Fwiw, I think it simply defaults to MyISAM, per:

http://dev.mysql.com/doc/refman/5.6/en/internal-temporary-tables.html

On a somewhat related note, regarding the MySQL page above, I filed a feature request with them to elaborate on that exact page about the new variable "default_tmp_storage_engine":

http://bugs.mysql.com/bug.php?id=73477

The gist of that is to describe the meaning of "default_tmp_storage_engine" on that page, since it is a new variable in MySQL 5.6, and very relevant to that page.

MariaDB has added this variable to MariaDB 10.1.0, so that is something to perhaps keep in mind when describing what OFF means:

https://mariadb.atlassian.net/browse/MDEV-6107

Not to mention, I've requested we backport this variable to 10.0 also, though not sure if this idea will be accepted or not.