Clean up VQL reference

[predictiple]

• Reduced inline accessor descriptions to basic descriptions since the full descriptions, including examples, are now represented in vql.yaml (which I double-checked).
• Standardized the markdown headings in vql.yaml descriptions so that they will look consistent on the documentation site.
• Fixed some minor issues in vql.yaml descriptions.
• Slightly changed formatting for accessors in vql list command so that they render prettier when piped through glow.
  
  Maybe in future we could do direct markdown rendering using Charm's Glamour package?