Fix general formatting issues found across design guidelines

[mmenestr]

• Divider
  • Add variations headings for vertical vs horizontal
• Data list
  • There’s a “when to use” under variations which shouldn’t be an H3
• File upload
  • When to use should be H3s
• Forms
  • Usage is after variations
• Overflow menu
  • Says “Content guidelines” but should be content considerations? And sentence case
• Popover
  • Behavior should be before variations
• Truncate
  • Should make variations H3