pgsql: Fix make maintainer-clean with queryjumblefuncs.*.c files in src

Fix make maintainer-clean with queryjumblefuncs.*.c files in src/backend/nodes/

The files generated by gen_node_support.pl for query jumbling
(queryjumblefuncs.funcs.c and queryjumblefuncs.switch.c) were not being
removed on make maintainer-clean (they need to remain around after a
simple "clean"). This commit makes the operation consistent with the
copy, equal, out and read files.

While on it, update a comment in the nodes'README where a reference to
queryjumblefuncs.funcs.c was missing.

Reported-by: Nathan Bossart
Reviewed-by: Richard Guo, Daniel Gustafsson
Discussion: https://postgr.es/m/ZBgAfTHcL6W7zGdW@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e8e1f96c497bda50ec02ed7360a6ed56c5f70f60

Modified Files
--------------
src/backend/nodes/Makefile | 3 ++-
src/backend/nodes/README | 8 ++++----
2 files changed, 6 insertions(+), 5 deletions(-)