DOCS: correct notes on TEMP_FOLDER

getTempParentFolder() in framewor/core/TempPath.php checks for silverstripe-cache directory in webroot first, failing that it falls back on the sys_get_temp_dir() folder.

also, getTempFolder() is no longer in framework/core/Core.php since #b075fa29c59f970bea31bbe8be1bd6560a8778b6, it is now located in framework/core/TempPath.php