More intuitive section/order handling

[dbarnett]

It's annoying how you don't have to specify section order with the @order directive until you add the first custom section, at which point you have to explicitly list every section ID in the order. You have to cross-reference the list of default sections if you want to preserve the default sections and order.

Also, if you have redundant definitions for the same section, vimdoc should complain instead of just picking one at random.