Some curve modifiers (namely Hook, SoftBody and MeshDeform) can only work on pre-tesselated spline points.

Before the modifier UI refactor in rB9b099c86123f, users would get the
'Apply on Spline' option, but disabled and with a tip explaining why
this cant be changed. After rB9b099c86123f though, this button was
always enabled [but disfunctional] leaving the user without an
explanation why this has no effect.

Now restore this functionality since it is quite important information.
Additionally, this button now appears to be ON in these cases which
makes more sense from the user perspective (so it does not represent the
actual setting on the modifier -- this would internally be switched ON in the modifier calculation anyways though, see
'curve_get_tessellate_point')