cross-posting to -hackers now that it is here.
v19 plans aside the existing wording is incorrect (just here, other places correctly point out the either-or aspect of the output).
The wording really should be more like:
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index fea683cb49..c545fee6c9 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -7614,10 +7614,11 @@ local0.* /var/log/postgresql
</term>
<listitem>
<para>
- By default, connection log messages only show the IP address of the
- connecting host. Turning this parameter on causes logging of the
- host name as well. Note that depending on your host name resolution
- setup this might impose a non-negligible performance penalty.
+ By default, for TCP/IP-originated connections, the identifier of the host
+ making the connection is its IP address. Turning this parameter on tells
+ the system to record the resolved host name instead. Note that depending
+ on your host name resolution setup this might impose a non-negligible
+ performance penalty.
This parameter can only be set in the <filename>postgresql.conf</filename>
file or on the server command line.
</para>
Both the connection logging routine and log_line_prefix %h / %r simply report the "identifier of the host making the connection".
David J.
