doc: Improve wording

Discussion: https://www.postgresql.org/message-id/flat/161626776179.652.11944895442156126506%40wrigleys.postgresql.org

doc/src/sgml/dml.sgml

@@ -26,9 +26,9 @@
   <para>
    When a table is created, it contains no data.  The first thing to
    do before a database can be of much use is to insert data.  Data is
-   conceptually inserted one row at a time.  Of course you can also
-   insert more than one row, but there is no way to insert less than
-   one row.  Even if you know only some column values, a
+   inserted one row at a time.  You can also insert more than one row
+   in a single command, but it is not possible to insert something that
+   is not a complete row.  Even if you know only some column values, a
    complete row must be created.
   </para>