Format list of catalog files in makefile vertically

I propose to reformat the catalog lists in src/backend/catalog/Makefile
to be more vertical, one per line. This makes it easier to keep that
list in sync with src/include/catalog/meson.build, and visually compare
both lists. Also, it's easier to read and edit in general.

In passing, I'd also copy over some relevant comments from the makefile
to meson.build. For the hypothetical future when we delete the
makefiles, these comments seem worth keeping. (For fun, I tested
whether the comments are still true, and yes, the order still matters.)