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

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