doc: Improve documentation related to table partitioning feature.

Robert Haas <rhaas@postgresql.org>

Commit: a1a4459c299a86f909c27e391a10d7b9b05ea697
Author: Robert Haas <rhaas@postgresql.org>
Date: 2016-12-13T13:18:00Z
Releases: 10.0
doc: Improve documentation related to table partitioning feature.

Commit f0e44751d7175fa3394da2c8f85e3ceb3cdbfe63 implemented table
partitioning, but failed to mention the "no row movement"
restriction in the documentation.  Fix that and a few other issues.

Amit Langote, with some additional wordsmithing by me.

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