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

Author: alvherre

doc/src/sgml/func.sgml

@@ -21450,6 +21450,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     (mtime) of each file in the log directory. By default, only superusers
     and members of the <literal>pg_monitor</literal> role can use this function.
     Access may be granted to others using <command>GRANT</command>.
+    Filenames beginning with a dot, directories, and other special files are not shown.
    </para>
 
    <indexterm>
@@ -21461,6 +21462,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     default only superusers and members of the <literal>pg_monitor</literal> role
     can use this function. Access may be granted to others using
     <command>GRANT</command>.
+    Filenames beginning with a dot, directories, and other special files are not shown.
    </para>
 
    <indexterm>
@@ -21473,6 +21475,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     superusers and members of the <literal>pg_monitor</literal> role can
     use this function. Access may be granted to others using
     <command>GRANT</command>.
+    Filenames beginning with a dot, directories, and other special files are not shown.
    </para>
 
    <indexterm>
@@ -21486,6 +21489,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     default only superusers and members of the <literal>pg_monitor</literal>
     role can use this function.  Access may be granted to others using
     <command>GRANT</command>.
+    Filenames beginning with a dot, directories, and other special files are not shown.
    </para>
 
    <indexterm>