Added an inverse transition function for array_agg()
Added an inverse transition function for string_agg
Merge branch 'invtrans' of https://github.com/david-rowley/postgres into invtrans
Forgot to specify the inverse transition function for string_agg(bytea) in pg_aggregate.h
Tried to massage the explanation of the expanded STRICT and NULL
behaviour of aggreages with inverse transition functions into the
existing description these cases for aggregates without inverse
transition functions.