From LORE (emphasis added):
When Power Attack is selected, it grants a +5 bonus to the damage roll
but at the cost of -5 to the attack roll on all melee attacks.
If you have a way to say it that is more clear, I'd be happy to listen, but unless I'm missing something, I think it states the way it works accurately (I'm not sure if you intended to imply this or not, but how you've phrased it is inaccurate, since the feat works with unarmed combat as well as melee weapons).
Edit: Perhaps you were looking at the improved power attack page? It did not state the limitation, and I just changed it.