xdg-activation: clarify what surfaces are expected as requestor of activation

While implementing support for xdg-activation in Chromium [1], we've realized that the spec is not clear about what types of surface should be used as the requestor surface, even though all the implementations of xdg_activation_v1 known atm accept the currently keyboard focused surface for activation, it'd be desirable to have this explicitly mentioned in the spec.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/4166961