pg_stat_activity: document client_port being null
As suggested by Stephen Frost.
Discussion:
@@ -652,7 +652,8 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry><structfield>client_port</structfield></entry>
<entry><type>integer</type></entry>
<entry>TCP port number that the client is using for communication
- with this backend, or <literal>-1</literal> if a Unix socket is used
+ with this backend, or <literal>-1</literal> if a Unix socket is used.
+ If this field is null, it indicates that this is an internal server process.
</entry>
</row>
<row>
