doc: Improve description of pg_ctl -l log file permissions

doc: Improve description of pg_ctl -l log file permissions

The documentation stated only that the log file created by pg_ctl -l is
inaccessible to other users by default. However, since commit c37b3d0,
the actual behavior is that only the cluster owner has access by default,
but users in the same group as the cluster owner may also read the file
if group access is enabled in the cluster.

This commit updates the documentation to describe this behavior
more clearly.

Backpatch to all supported versions.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Reviewed-by: Xiaopeng Wang <wxp_728@163.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/OS9PR01MB1214959BE987B4839E3046050F54BA@OS9PR01MB12149.jpnprd01.prod.outlook.com
Backpatch-through: 14