pgsql: Improve description of SPI_scroll_cursor_move's return values,

Log Message:
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Improve description of SPI_scroll_cursor_move's return values, per a
gripe by Pavel Stehule awhile ago.

Modified Files:
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pgsql/doc/src/sgml:
spi.sgml (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/spi.sgml.diff?r1=1.57&r2=1.58)