Adjust documentation with respect to "unknown" timezone setting.

The recent cleanup of GUC assign hooks got rid of the kludge of using
"unknown" as a magic value for timezone and log_timezone.  But I forgot
to update the documentation to match, as noted by Martin Pitt.

doc/src/sgml/config.sgml

@@ -3869,11 +3869,11 @@ FROM pg_stat_activity;
       </indexterm>
       <listitem>
        <para>
-        Sets the time zone used for timestamps written in the log.
+        Sets the time zone used for timestamps written in the server log.
         Unlike <xref linkend="guc-timezone">, this value is cluster-wide,
         so that all sessions will report timestamps consistently.
-        The default is <literal>unknown</>, which means use whatever
-        the system environment specifies as the time zone.  See <xref
+        If not explicitly set, the server initializes this variable to the
+        time zone specified by its system environment.  See <xref
         linkend="datatype-timezones"> for more information.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
@@ -4918,10 +4918,9 @@ SET XML OPTION { DOCUMENT | CONTENT };
       <listitem>
        <para>
         Sets the time zone for displaying and interpreting time stamps.
-        The default is <literal>unknown</>, which means to use whatever
-        the system environment specifies as the time zone.  See <xref
-        linkend="datatype-timezones"> for more
-        information.
+        If not explicitly set, the server initializes this variable to the
+        time zone specified by its system environment.  See <xref
+        linkend="datatype-timezones"> for more information.
        </para>
       </listitem>
      </varlistentry>

src/backend/utils/misc/postgresql.conf.sample

@@ -390,8 +390,7 @@
 #log_temp_files = -1			# log temporary files equal or larger
 					# than the specified size in kilobytes;
 					# -1 disables, 0 logs all temp files
-#log_timezone = unknown			# actually, defaults to TZ environment
-					# setting
+#log_timezone = '(defaults to server environment setting)'
 
 
 #------------------------------------------------------------------------------
@@ -471,8 +470,7 @@
 
 #datestyle = 'iso, mdy'
 #intervalstyle = 'postgres'
-#timezone = unknown			# actually, defaults to TZ environment
-					# setting
+#timezone = '(defaults to server environment setting)'
 #timezone_abbreviations = 'Default'     # Select the set of available time zone
 					# abbreviations.  Currently, there are
 					#   Default