[TYPO3-core] Fix #11669: Provide a reason for why $TSFE->set_no_cache() has been triggered

[Steffen Kamper]

Hi Michael,

+1 by reading (beside some spaces missing)

I would be more radical and would make the param not optional. it would 
break extensions doing this and help finding those evils, as we know 
that there is always a better way doing this. But maybe it's too radical.

vg Steffen