pgsql: 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(at)telsasoft(dot)com>
Discussion: https://postgr.es/m/aj6BPoziSb-F8aJz%40pryzbyj2023
Reported-by: Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/78860fab26f1440ec4a8e31d039a88c66329196f

Modified Files
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doc/src/sgml/ref/alter_table.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)