Clarify the result order of unnest(multirange).

Clarify the result order of unnest(multirange).

It is best not to mention the storage order, because that is
an implementation detail and has confused at least one user,
who assumed that the storage order is the order in which the
constituent ranges were written in SQL.

Since the sorting order is explained at the beginning of the
page, it should be sufficient to say that the ranges are
returned in ascending order.

Author: Laurenz Albe
Reviewed-by: Daniel Fredouille
Discussion: https://postgr.es/m/169627213477.3727338.17653654241633692682%40wrigleys.postgresql.org