pgsql: Fix pg_stat_statements for MERGE

Fix pg_stat_statements for MERGE

We weren't jumbling the merge action list, so wildly different commands
would be considered to use the same query ID.  Add that, mention it in
the docs, and some test lines.

Backpatch to 15.

Author: Tatsu <bt22nakamorit@oss.nttdata.com>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/d87e391694db75a038abc3b2597828e8@oss.nttdata.com

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/72abf03b6491a8df880e1fea45798797bcc86c47

Modified Files
--------------
.../expected/pg_stat_statements.out                | 41 +++++++++++++++++++++-
.../pg_stat_statements/sql/pg_stat_statements.sql  | 22 ++++++++++++
doc/src/sgml/pgstatstatements.sgml                 |  4 +--
src/backend/nodes/nodeFuncs.c                      |  4 +--
src/backend/utils/misc/queryjumble.c               | 11 ++++++
5 files changed, 77 insertions(+), 5 deletions(-)