pgsql: Fix SPI documentation for new handling of ExecutorRun's count pa

Fix SPI documentation for new handling of ExecutorRun's count parameter.

Since 9.0, the count parameter has only limited the number of tuples
actually returned by the executor. It doesn't affect the behavior of
INSERT/UPDATE/DELETE unless RETURNING is specified, because without
RETURNING, the ModifyTable plan node doesn't return control to execMain.c
for each tuple. And we only check the limit at the top level.

While this behavioral change was unintentional at the time, discussion of
bug #6572 led us to the conclusion that we prefer the new behavior anyway,
and so we should just adjust the docs to match rather than change the code.
Accordingly, do that. Back-patch as far as 9.0 so that the docs match the
code in each branch.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/49e0ea5991bf00a68e753583738a6759b7b29f2d

Modified Files
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doc/src/sgml/spi.sgml | 47 +++++++++++++++++++++++++-------------
src/backend/executor/execMain.c | 6 +++-
2 files changed, 35 insertions(+), 18 deletions(-)