spec should be more consistent with cycle versus overflow terminology

[dbaron]

In the definition of toggle-root a concept is called <toggle-overflow> but in the API this same toggle is called "cycle" (in the cycle attribute on CSSToggle, and in the CSSToggleCycle enum).

I think these names should probably be consistent rather than using 2 different names for the same thing. I might slightly lean towards overflow since then the concept has a name different from its values.