Add description about LogicalRewriteTruncate wait event into document.

Back-patch to v10 where commit 249cf070 introduced
LogicalRewriteTruncate wait event.

Author: Fujii Masao
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/949931aa-4ed4-d867-a7b5-de9c02b2292b@oss.nttdata.com

doc/src/sgml/monitoring.sgml

@@ -1493,7 +1493,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry>Waiting to apply WAL at recovery because it is delayed.</entry>
         </row>
         <row>
-         <entry morerows="67"><literal>IO</literal></entry>
+         <entry morerows="68"><literal>IO</literal></entry>
          <entry><literal>BufFileRead</literal></entry>
          <entry>Waiting for a read from a buffered file.</entry>
         </row>
@@ -1609,6 +1609,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>LogicalRewriteSync</literal></entry>
          <entry>Waiting for logical rewrite mappings to reach stable storage.</entry>
         </row>
+        <row>
+         <entry><literal>LogicalRewriteTruncate</literal></entry>
+         <entry>Waiting for truncate of mapping data during a logical rewrite.</entry>
+        </row>
         <row>
          <entry><literal>LogicalRewriteWrite</literal></entry>
          <entry>Waiting for a write of logical rewrite mappings.</entry>