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

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