Discourage splitting pg_wal directory

Someone pointed out this morning that we still have this in our docs:

"It is advantageous if the WAL is located on a different disk from the main
database files. This can be achieved by moving the pg_wal directory to
another location (while the server is shut down, of course) and creating a
symbolic link from the original location in the main data directory to the
new location."

This is not as important as it used to be, and I would even hazard to say
that we should not be encouraging it. There are still use cases for doing
so, yes, but "advantageous" is too much. I played with some wording, but at
the end of the day, I am thinking that we simply remove this paragraph
entirely.

Cheers,
Greg