pgsql: Add missing support for the latest SPI status codes.

Add missing support for the latest SPI status codes.

SPI_result_code_string() was missing support for SPI_OK_TD_REGISTER,
and in v15 and later, it was missing support for SPI_OK_MERGE, as was
pltcl_process_SPI_result().

The last of those would trigger an error if a MERGE was executed from
PL/Tcl. The others seem fairly innocuous, but worth fixing.

Back-patch to all supported branches. Before v15, this is just adding
SPI_OK_TD_REGISTER to SPI_result_code_string(), which is unlikely to
be seen by anyone, but seems worth doing for completeness.

Reviewed by Tom Lane.

Discussion:
  https://postgr.es/m/CAEZATCUg8V%2BK%2BGcafOPqymxk84Y_prXgfe64PDoopjLFH6Z0Aw%40mail.gmail.com
  https://postgr.es/m/CAEZATCUMe%2B_KedPMM9AxKqm%3DSZogSxjUcrMe%2BsakusZh3BFcQw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0460a31de6acc8bbb86e9e3d646f1113ebb0c20

Modified Files
--------------
doc/src/sgml/spi.sgml      | 9 +++++++++
src/backend/executor/spi.c | 4 ++++
src/pl/tcl/pltcl.c         | 1 +
3 files changed, 14 insertions(+)