Avoid "ATX" and "setext" for heading types

[Crissov]

Many spec authors and implementers may be used to the terms ATX heading and setext heading by now, but they are completely opaque to any newbie who has never worked with either system before. Some people, like myself, do know both terms but confuse them all the time. Can we please adopt more descriptive aliases, e.g. prefixed heading and underlined heading?