[BUGFIX] Add description

Add the information, that the extension is not class loader
or a PHP SPL autoloader.

Resolves: #61846
Change-Id: Ic6344eef488d506ce1eb707e6b2978dde0f7985c
Reviewed-on: http://review.typo3.org/32964
Reviewed-by: Tim Lochmüller <tim@fruit-lab.de>
Tested-by: Tim Lochmüller <tim@fruit-lab.de>