pgsql: Fixes for SPI "const Datum *" use

Fixes for SPI "const Datum *" use

Fixup for commit 8a27d418f8f, which converted many functions to use
"const Datum *" instead of "Datum *", including some SPI functions.

For SPI_cursor_open(), the code was updated but not the documentation.
For SPI_cursor_open_with_args(), the documentation was updated but not
the code. (Possibly, these two were confused with each other.) Also,
SPI_execp() and SPI_modifytuple() were not updated, even though they
are closely related to the functions touched by the previous commit
and now look inconsistent. Fix all these.

Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://www.postgresql.org/message-id/flat/86b5162f-c472-40fa-997b-0450dece1dec%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cd3ad3bc03567ee120a638c840112b8865e055a8

Modified Files
--------------
doc/src/sgml/spi.sgml | 6 +++---
src/backend/executor/spi.c | 6 +++---
src/include/executor/spi.h | 6 +++---
3 files changed, 9 insertions(+), 9 deletions(-)