core: fixing taskcleaner.sh utility help

This patch fixes the -R option description to note that all tasks will
be removed unless also the -z option is given.

Change-Id: If139423d1158060b00d0c205bc94608a06e1572c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1084774
Signed-off-by: Eli Mesika <emesika@redhat.com>