Spellcheck: s/descendent/descendant/g

I got a little annoyed by reading documentation paragraphs containing
both spellings within a few lines of each other.  My dictionary says
"descendant" is the preferred spelling, and it's certainly the majority
usage in our tree, so standardize on that.

For one usage in parallel.sgml, I thought it better to rewrite to avoid
the term altogether.

doc/src/sgml/parallel.sgml

@@ -47,8 +47,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     child plan, which is the portion of the plan that will be executed in
     parallel.  If the <literal>Gather</> node is at the very top of the plan
     tree, then the entire query will execute in parallel.  If it is somewhere
-    else in the plan tree, then only that portion of the query will run in
-    parallel.  In the example above, the query accesses only one table, so
+    else in the plan tree, then only the portion of the plan below it will run
+    in parallel.  In the example above, the query accesses only one table, so
     there is only one plan node other than the <literal>Gather</> node itself;
     since that plan node is a child of the <literal>Gather</> node, it will
     run in parallel.
@@ -76,8 +76,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
 
    <para>
     Every background worker process which is successfully started for a given
-    parallel query will execute the portion of the plan which is a descendent
-    of the <literal>Gather</> node.  The leader will also execute that portion
+    parallel query will execute the portion of the plan below
+    the <literal>Gather</> node.  The leader will also execute that portion
     of the plan, but it has an additional responsibility: it must also read
     all of the tuples generated by the workers.  When the parallel portion of
     the plan generates only a small number of tuples, the leader will often

doc/src/sgml/queries.sgml

@@ -146,7 +146,7 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
     Instead of writing <literal>ONLY</> before the table name, you can write
     <literal>*</> after the table name to explicitly specify that descendant
     tables are included.  There is no real reason to use this syntax any more,
-    because searching descendent tables is now always the default behavior.
+    because searching descendant tables is now always the default behavior.
     However, it is supported for compatibility with older releases.
    </para>
 

doc/src/sgml/ref/alter_table.sgml

@@ -1116,11 +1116,11 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
     rename, or change the type of a column in the parent table without doing
     same to the descendants.  This ensures that the descendants always have
     columns matching the parent.  Similarly, a constraint cannot be renamed
-    in the parent without also renaming it in all descendents, so that
-    constraints also match between the parent and its descendents.
-    Also, because selecting from the parent also selects from its descendents,
+    in the parent without also renaming it in all descendants, so that
+    constraints also match between the parent and its descendants.
+    Also, because selecting from the parent also selects from its descendants,
     a constraint on the parent cannot be marked valid unless it is also marked
-    valid for those descendents.  In all of these cases, <command>ALTER TABLE
+    valid for those descendants.  In all of these cases, <command>ALTER TABLE
     ONLY</command> will be rejected.
    </para>
 

src/backend/executor/execParallel.c

@@ -518,7 +518,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 }
 
 /*
- * Copy instrumentation information about this node and its descendents from
+ * Copy instrumentation information about this node and its descendants from
  * dynamic shared memory.
  */
 static bool
@@ -666,7 +666,7 @@ ExecParallelGetQueryDesc(shm_toc *toc, DestReceiver *receiver,
 }
 
 /*
- * Copy instrumentation information from this node and its descendents into
+ * Copy instrumentation information from this node and its descendants into
  * dynamic shared memory, so that the parallel leader can retrieve it.
  */
 static bool
@@ -706,7 +706,7 @@ ExecParallelReportInstrumentation(PlanState *planstate,
 }
 
 /*
- * Initialize the PlanState and its descendents with the information
+ * Initialize the PlanState and its descendants with the information
  * retrieved from shared memory.  This has to be done once the PlanState
  * is allocated and initialized by executor; that is, after ExecutorStart().
  */