Stop documenting heading eyebrows

ADR-046 decides that an eyebrow is a paragraph directly before the headline, never a heading. The docs still teach the heading form. The guide's only example of it uses hero, which renders it wrongly (BUG-047), and nineteen rune pages say "a short paragraph or heading" in the same boilerplate sentence.

The native page-section runes keep their grammar until each one is composed (ADR-046, decision 4), so some of them still accept the heading form. The docs stop recommending it anyway: it is not the direction, and documenting less than a rune accepts is safe where documenting more is not.