doc: clarify MERGE PARTITIONS adjacency requirement

doc: clarify MERGE PARTITIONS adjacency requirement

The existing description says the ranges of merged range-partitions
"must be adjacent" only under the heading "If the DEFAULT partition is
not in the list of merged partitions".  That could be misread as
a restriction tied to the presence of a default partition.  In fact,
merging non-adjacent ranges is rejected regardless of whether
the partitioned table has a default partition; spell that out explicitly.

Also, this commit removes a small redundancy in the documentation sentence
stating that "merged range-partitions" are "to be merged".

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/aj6BPoziSb-F8aJz%40pryzbyj2023
Reported-by: Pavel Borisov <pashkin.elfe@gmail.com>
Backpatch-through: 19