pgsql: Docs: improve CREATE TABLE ref page's discussion of partition bo

Docs: improve CREATE TABLE ref page's discussion of partition bounds.

Clarify in the syntax synopsis that partition bound values must be
exactly numeric literals or string literals; previously it
said "bound_literal" which was defined nowhere.

Replace confusing --- and, I think, incorrect in detail --- definition
of how range bounds work with a reference to row-wise comparison plus
a concrete example (which I stole from Robert Haas).

Minor copy-editing in the same area.

Discussion: https://postgr.es/m/30475.1496005465@sss.pgh.pa.us
Discussion: https://postgr.es/m/28106.1496041449@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0198c277a29a035aa8a4e6767967201135f6caa9

Modified Files
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doc/src/sgml/ref/create_table.sgml | 72 ++++++++++++++++++++++----------------
1 file changed, 41 insertions(+), 31 deletions(-)