Add missing dblink files.

contrib/dblink/doc/execute

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+==================================================================
+Name
+
+dblink_exec -- Executes an UPDATE/INSERT/DELETE on a remote database
+
+Synopsis
+
+dblink_exec(text connstr, text sql)
+- or -
+dblink_exec(text sql)
+
+Inputs
+
+  connstr
+
+    standard libpq format connection string, 
+    e.g. "hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd"
+    If the second form is used, then the dblink_connect(text connstr) must be
+    executed first.
+
+  sql
+
+    sql statement that you wish to execute on the remote host, e.g.:
+
+       insert into foo values(0,'a','{"a0","b0","c0"}');
+
+Outputs
+
+  Returns status of the command
+
+Notes
+  1) dblink_open starts an explicit transaction. If, after using dblink_open,
+     you use dblink_exec to change data, and then an error occurs or you use
+     dblink_disconnect without a dblink_close first, your change *will* be
+     lost.
+
+Example usage
+
+test=# select dblink_connect('dbname=dblink_test_slave');
+ dblink_connect
+----------------
+ OK
+(1 row)
+
+test=# select dblink_exec('insert into foo values(21,''z'',''{"a0","b0","c0"}'');');
+   dblink_exec
+-----------------
+ INSERT 943366 1
+(1 row)