doc: Reword old inheritance partitioning documentation
doc: Reword old inheritance partitioning documentation

Prefer to use phrases like "child" instead of "partition" when
describing the legacy inheritance-based partitioning.  The word
"partition" now has a fixed meaning for the built-in partitioning, so
keeping it out of the documentation of the old method makes things
clearer.

Author: Justin Pryzby <ca289b655fccb1d52de932b0853f82a8386c7d66@telsasoft.com>

greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb