Doc: use book-style capitalization for headings

Change-Id: I8a406c85ecda29b2f16dfd184dafd5a1053ba2b0
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>