doc: Clarify status of SELECT INTO on reference page

 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