doc: Clarify status of SELECT INTO on reference page

The documentation as well as source code comments weren't entirely
clear whether SELECT INTO was truly deprecated (thus in theory
destined to be removed eventually), or just a less recommended
variant.  After discussion, it appears that other implementations also
use SELECT INTO in direct SQL in a way similar to PostgreSQL, so it
seems worth keeping it for compatibility.  Update the language in the
documentation to that effect.

Discussion: https://www.postgresql.org/message-id/flat/96dc0df3-e13a-a85d-d045-d6e2c85218da%40enterprisedb.com

doc/src/sgml/ref/select_into.sgml

@@ -138,9 +138,11 @@ SELECT * INTO films_recent FROM films WHERE date_prod >= '2002-01-01';
    in <application>ECPG</application> (see <xref linkend="ecpg"/>) and
    <application>PL/pgSQL</application> (see <xref linkend="plpgsql"/>).
    The <productname>PostgreSQL</productname> usage of <command>SELECT
-   INTO</command> to represent table creation is historical.  It is
-   best to use <command>CREATE TABLE AS</command> for this purpose in
-   new code.
+   INTO</command> to represent table creation is historical.  Some other SQL
+   implementations also use <command>SELECT INTO</command> in this way (but
+   most SQL implementations support <command>CREATE TABLE AS</command>
+   instead).  Apart from such compatibility considerations, it is best to use
+   <command>CREATE TABLE AS</command> for this purpose in new code.
   </para>
  </refsect1>