Fix collection of typos and grammar mistakes in the tree

This fixes some comments and documentation new as of Postgres 13.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20200408165653.GF2228@telsasoft.com

src/backend/commands/tablecmds.c

@@ -16846,7 +16846,7 @@ ATExecDetachPartition(Relation rel, RangeVar *name)
 	list_free_deep(fks);
 
 	/*
-	 * Any sub-constrains that are in the referenced-side of a larger
+	 * Any sub-constraints that are in the referenced-side of a larger
 	 * constraint have to be removed.  This partition is no longer part of the
 	 * key space of the constraint.
 	 */

src/backend/executor/execExpr.c

@@ -2366,7 +2366,7 @@ get_last_attnums_walker(Node *node, LastAttnumInfo *info)
  * evaluation of the expression will have the same type of slot, with an
  * equivalent descriptor.
  *
- * Returns true if the the deforming step is required, false otherwise.
+ * Returns true if the deforming step is required, false otherwise.
  */
 static bool
 ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)

src/backend/postmaster/bgworker.c

@@ -1149,7 +1149,7 @@ WaitForBackgroundWorkerShutdown(BackgroundWorkerHandle *handle)
  * Instruct the postmaster to terminate a background worker.
  *
  * Note that it's safe to do this without regard to whether the worker is
- * still running, or even if the worker may already have existed and been
+ * still running, or even if the worker may already have exited and been
  * unregistered.
  */
 void

src/backend/replication/basebackup.c

@@ -1157,7 +1157,7 @@ AddFileToManifest(manifest_info *manifest, const char *spcoid,
 	}
 
 	/*
-	 * Each file's entry need to be separated from any entry that follows by a
+	 * Each file's entry needs to be separated from any entry that follows by a
 	 * comma, but there's no comma before the first one or after the last one.
 	 * To make that work, adding a file to the manifest starts by terminating
 	 * the most recently added line, with a comma if appropriate, but does not