Clarify SPLIT PARTITION bound requirements in docs

Clarify SPLIT PARTITION bound requirements in docs

The documentation said that the bounds of new partitions should not
overlap and that their combined bounds should equal the bounds of the
split partition.  That is misleading when a new DEFAULT partition is
specified, because the explicit partitions may cover only part of the
split partition while the DEFAULT partition covers the rest.

Clarify that new non-DEFAULT partition bounds must not overlap with
other new or existing partitions and must be contained within the bounds
of the split partition.  Also state that the combined bounds must exactly
match the split partition only when no new DEFAULT partition is specified.

While here, improve nearby wording about hash-partitioned target tables
and splitting a DEFAULT partition with the same partition name.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Discussion: https://postgr.es/m/C18878AB-DEB2-4A61-9995-A035DD644B81@gmail.com