RFC template: move "Drawbacks" section after "Detailed design"

Having "Drawbacks" earlier in the document than "Detailed design" makes
it awkward to discuss drawbacks that reference details of the design.
This encourages authors to either violate causality by referencing
details before they are introduced or to neglect the "Drawbacks" section
entirely and just sprinkle the discussion of drawbacks throught
"Detailed design".