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@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/86b5162f-c472-40fa-997b-0450dece1dec%40eisentraut.org