Doc: fix outdated wording about parallel seq scans

56788d21 adjusted the parallel seq scan code so that instead of handing
out a single block at a time to parallel workers, it now hands out ranges
of blocks.

Here we update the documentation which still claimed that workers received
just 1 block at a time.

Reported-by: Zhang Mingli
Discussion: https://postgr.es/m/17c99615-2c3b-4e4e-9d0b-424a66a7bccd@Spark
Backpatch-through: 14, where 56788d21 was added.

doc/src/sgml/parallel.sgml

@@ -272,8 +272,9 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     <listitem>
       <para>
         In a <emphasis>parallel sequential scan</emphasis>, the table's blocks will
-        be divided among the cooperating processes.  Blocks are handed out one
-        at a time, so that access to the table remains sequential.
+        be divided into ranges and shared among the cooperating processes.  Each
+        worker process will complete the scanning of its given range of blocks before
+        requesting an additional range of blocks.
       </para>
     </listitem>
     <listitem>