Document state_agg changes in Toolkit 1.14

[syvb]

Description

Documents the changes to state_agg in Toolkit 1.14:

• Renamed state_agg → compact_state_agg; renamed timeline_agg → state_agg
• Removed next parameter from all interpolated state_agg functions
• Added next and interval parameters to duration_in
• Swapped arguments to {interpolated_,}{duration_in,state_periods} so that the aggregate is always first and the state is always second
• Made into_[int_]values return INTERVALs instead of a BIGINT representing the number of microseconds in the state

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