elaborate on what is required for using the tempdirectory setting of nextcloud

Signed-off-by: Florian Goth <fgoth@physik.uni-wuerzburg.de>

config/config.sample.php

@@ -1840,7 +1840,10 @@ $CONFIG = [
  * restricted, or if external storage which do not support streaming are in
  * use.
  *
- * The Web server user must have write access to this directory.
+ * The Web server user/PHP must have write access to this directory.
+ * Additionally you have to make sure, that you php configuration considers this a valid
+ * tmp directory, by setting the TMP, TMPDIR, and TEMP variables to the required directories.
+ * On top of that you might be required to grant additional permissions in AppArmor or SELinux.
  */
 'tempdirectory' => '/tmp/nextcloudtemp',