Fix CLogTruncationLock documentation.

Back-patch to v10, which introduced the lock.

Author: nmisch

doc/src/sgml/monitoring.sgml

@@ -1040,7 +1040,8 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
         </row>
         <row>
          <entry><literal>CLogTruncationLock</literal></entry>
-         <entry>Waiting to truncate the write-ahead log or waiting for write-ahead log truncation to finish.</entry>
+         <entry>Waiting to execute <function>txid_status</function> or update
+         the oldest transaction id available to it.</entry>
         </row>
         <row>
          <entry><literal>clog</literal></entry>