doc: Mention that partitions_{done,total} is 0 for REINDEX progress reports

REINDEX has recently gained support for partitions, so it can be
confusing to see those fields not being set.  Making useful reports for
for such relations is more complicated than it looks with the current
set of columns available in pg_stat_progress_create_index, and this
touches equally REINDEX DATABASE/SYSTEM/SCHEMA.  This commit documents
that those two columns are not touched during a REINDEX.

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/20210216064214.GI28165@telsasoft.com

doc/src/sgml/monitoring.sgml

@@ -5716,6 +5716,7 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
       <para>
        When creating an index on a partitioned table, this column is set to
        the total number of partitions on which the index is to be created.
+       This field is <literal>0</literal> during a <literal>REINDEX</literal>.
       </para></entry>
      </row>
 
@@ -5726,6 +5727,7 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
       <para>
        When creating an index on a partitioned table, this column is set to
        the number of partitions on which the index has been completed.
+       This field is <literal>0</literal> during a <literal>REINDEX</literal>.
       </para></entry>
      </row>
     </tbody>