Document pg_ls_*dir hiding of directories and special files

Document pg_ls_*dir hiding of directories and special files

It's strange that a directory-listing function does not list all entries
in a directory, so let's at least document it.  This involves

pg_ls_logdir
pg_ls_waldir
pg_ls_archive_statusdir
pg_ls_tmpdir

Backpatch as far back as it applies cleanly (and as far as as each
function exists).  REL_10_STABLE uses different wording, but hopefully
people are not reading docs so old to write new apps anyway.

Author: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20200305161838.GJ684@telsasoft.com