Documentation improvements for partitioning

Here are some patches to improve the documentation about partitioned tables:

0001: Adds some details about partition_bound_spec to the CREATE TABLE
page, especially:

- a note about inclusivity of range partition bounds,
- a note about the UNBOUNDED literal in case of range partitioning,
- a note about the NULL literal in case of list partitioning,

I wonder if the above "note" should be added under the Notes section or
are they fine to be added as part of the description of PARTITION OF
clause. Also:

- in syntax synopsis, it appears now that any expression is OK to be used
for individual bound datum, but it's not true. Only literals are
allowed. So fixed that.
- added an example showing how to create partitions of a range
partitioned table with multiple columns in the partition key
- added PARTITION BY and PARTITION OF (FOR VALUES) as PostgreSQL
extensions in the compatibility section

0002: Adds details about partitioned tables to the DDL chapter (ddl.sgml)

- a new section named "Partitioned Tables" right next to the
Inheritance and Partitioning sections is created.
- examples are added to the existing Partitioning section using the new
partitioned tables. Old text about implementing table partitioning
using inheritance is kept, sort of as a still supported older
alternative.

0003: Add partitioning keywords to keywords.sgml

This is all I have for now. Any feedback is greatly appreciated. Adding
this to the next CF.

Thanks,
Amit