Huh. I wonder if a section went missing; seems like none of the extensions that alter or relax layout are documented currently. (AlternativeLayoutRule, AlternativeLayoutRuleTransitional, DoAndIfThenElse, NondecreasingIndentation, RelaxedLayout)
IIRC DoAndIfThenElse relaxes a condition implied by layout but that normally only matters in "do": that if you break it into multiple lines, the "then" and "else" must be indented farther than the "if" or layout will consider them distinct new expressions (and thereby syntax errors).