Improve documentation around logging_collector and use of stderr.

In backup.sgml, point out that you need to be using the logging collector
if you want to log messages from a failing archive_command script.  (This
is an oversimplification, in that it will work without the collector as
long as you're not sending postmaster stderr to /dev/null; but it seems
like a good idea to encourage use of the collector to avoid problems
with multiple processes concurrently scribbling on one file.)

In config.sgml, do some wordsmithing of logging_collector discussion.

Per bug #6518 from Janning Vygen

doc/src/sgml/backup.sgml

@@ -1277,9 +1277,6 @@ archive_command = 'local_backup_script.sh "%p" "%f"'
       This allows all complexity to be managed within the script, which
       can be written in a popular scripting language such as
       <application>bash</> or <application>perl</>.
-      Any messages written to <literal>stderr</> from the script will appear
-      in the database server log, allowing complex configurations to be
-      diagnosed easily if they fail.
      </para>
 
      <para>
@@ -1308,6 +1305,16 @@ archive_command = 'local_backup_script.sh "%p" "%f"'
        </listitem>
       </itemizedlist>
      </para>
+
+     <tip>
+      <para>
+       When using an <varname>archive_command</varname> script, it's desirable
+       to enable <xref linkend="guc-logging-collector">.
+       Any messages written to <systemitem>stderr</> from the script will then
+       appear in the database server log, allowing complex configurations to
+       be diagnosed easily if they fail.
+      </para>
+     </tip>
     </sect3>
   </sect2>
 

doc/src/sgml/config.sgml

@@ -2961,7 +2961,7 @@ SELECT * FROM parent WHERE key = 2400;
         value</> (<acronym>CSV</>) format, which is convenient for
         loading logs into programs.
         See <xref linkend="runtime-config-logging-csvlog"> for details.
-        <varname>logging_collector</varname> must be enabled to generate
+        <xref linkend="guc-logging-collector"> must be enabled to generate
         CSV-format log output.
        </para>
 
@@ -2993,24 +2993,39 @@ local0.*    /var/log/postgresql
       </indexterm>
       <listitem>
        <para>
-         This parameter captures plain and CSV-format log messages
-         sent to <application>stderr</> and redirects them into log files.
+         This parameter enables the <firstterm>logging collector</>, which
+         is a background process that captures log messages
+         sent to <systemitem>stderr</> and redirects them into log files.
          This approach is often more useful than
          logging to <application>syslog</>, since some types of messages
-         might not appear in <application>syslog</> output (a common example
-         is dynamic-linker failure messages).
+         might not appear in <application>syslog</> output.  (One common
+         example is dynamic-linker failure messages; another is error messages
+         produced by scripts such as <varname>archive_command</>.)
          This parameter can only be set at server start.
        </para>
 
+       <note>
+        <para>
+         It is possible to log to <systemitem>stderr</> without using the
+         logging collector; the log messages will just go to wherever the
+         server's <systemitem>stderr</> is directed.  However, that method is
+         only suitable for low log volumes, since it provides no convenient
+         way to rotate log files.  Also, on some platforms not using the
+         logging collector can result in lost or garbled log output, because
+         multiple processes writing concurrently to the same log file can
+         overwrite each other's output.
+        </para>
+       </note>
+
        <note>
         <para>
           The logging collector is designed to never lose messages.  This means
           that in case of extremely high load, server processes could be
-          blocked due to trying to send additional log messages when the
+          blocked while trying to send additional log messages when the
           collector has fallen behind.  In contrast, <application>syslog</>
-          prefers to drop messages if it cannot write them, which means it's
-          less reliable in those cases but it will not block the rest of the
-          system.
+          prefers to drop messages if it cannot write them, which means it
+          may fail to log some messages in such cases but it will not block
+          the rest of the system.
         </para>
        </note>