Oh, it says it *somewhere * although maybe not entirely directly. the SR5 core rule book is notorious for burying key points deep in long blocks of text, and scattering the information you need on a given topic over multiple locations (just for fun, look up how spirits immunity to mundane weapons works)
OK, several of the cyber ware categories (headwaters, eyeware, bodyware, earmark) talk about capacity or essence. On page 456 in the cyber limbs section it states (at the end of a paragraph about agility and strength) "cyberlimbs cannot hold any bioware, nor any cyber-implant that takes up essence rather than capacity."
I think between the other sections and that sentence it makes it clear enough that weapons can use capacity or essence.