Improve documentation for table partitioning.

Emphasize the new declarative partitioning more, and compare and
contrast it more clearly with inheritance-based partitioning.

Amit Langote, reviewed and somewhat revised by me

Discussion: http://postgr.es/m/a6f99cdb-21e7-1d65-1381-91f2cfa156e2@lab.ntt.co.jp

Author: robertmhaas