Fix reference/structure/keyword.md
reference: Add mention of sealed
reference: AssertionException -> AssertionError
reference: Obtainable and Destroyable, and Throwable.
reference: Various misc improvements for the reference for statements.
reference: Various misc improvements for the reference for declarations.
reference: - defined on Invertiable not Numeric.
reference segment->measure (with new definition in terms of Enumerable).
reference: minor change since Invertible is a subtype of Summable.