pgsql: Add BETWEEN SYMMETRIC.

Log Message:
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Add BETWEEN SYMMETRIC.

Pavel Stehule

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.255 -> r1.256)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.255&r2=1.256)
    pgsql/src/backend/parser:
        gram.y (r2.492 -> r2.493)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.492&r2=2.493)
        keywords.c (r1.155 -> r1.156)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/keywords.c.diff?r1=1.155&r2=1.156)