doc: Clarify when SSL actually means TLS

SSL has become the de facto term to mean an end-to-end encrypted channel
regardless of protocol used, even though the SSL protocol is deprecated.
Clarify what we mean with SSL in our documentation, especially for new
users who might be looking for TLS.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/D4ABB281-6CFD-46C6-A4E0-8EC23A2977BC@yesql.se

doc/src/sgml/config.sgml

@@ -1184,7 +1184,13 @@ include_dir 'conf.d'
      <title>SSL</title>
 
      <para>
-      See <xref linkend="ssl-tcp"/> for more information about setting up SSL.
+      See <xref linkend="ssl-tcp"/> for more information about setting up
+      <acronym>SSL</acronym>. The configuration parameters for controlling
+      transfer encryption using <acronym>TLS</acronym> protocols are named
+      <literal>ssl</literal> for historic reasons, even though support for
+      the <acronym>SSL</acronym> protocol has been deprecated.
+      <acronym>SSL</acronym> is in this context used interchangeably with
+      <acronym>TLS</acronym>.
      </para>
 
      <variablelist>

doc/src/sgml/libpq.sgml

@@ -8292,12 +8292,14 @@ ldap://ldap.acme.com/cn=dbserver,cn=hosts?pgconnectinfo?base?(objectclass=*)
 
   <indexterm zone="libpq-ssl">
    <primary>SSL</primary>
+   <secondary>TLS</secondary>
   </indexterm>
 
   <para>
    <productname>PostgreSQL</productname> has native support for using <acronym>SSL</acronym>
-   connections to encrypt client/server communications for increased
+   connections to encrypt client/server communications using
-   security. See <xref linkend="ssl-tcp"/> for details about the server-side
+   <acronym>TLS</acronym> protocols for increased security.
+   See <xref linkend="ssl-tcp"/> for details about the server-side
    <acronym>SSL</acronym> functionality.
   </para>
 

doc/src/sgml/runtime.sgml

@@ -2182,6 +2182,7 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
 
   <indexterm zone="ssl-tcp">
    <primary>SSL</primary>
+   <secondary>TLS</secondary>
   </indexterm>
 
   <para>
@@ -2193,13 +2194,25 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
    enabled at build time (see <xref linkend="installation"/>).
   </para>
 
+  <para>
+   The terms <acronym>SSL</acronym> and <acronym>TLS</acronym> are often used
+   interchangeably to mean a secure encrypted connection using a
+   <acronym>TLS</acronym> protocol. <acronym>SSL</acronym> protocols are the
+   precursors to <acronym>TLS</acronym> protocols, and the term
+   <acronym>SSL</acronym> is still used for encrypted connections even though
+   <acronym>SSL</acronym> protocols are no longer supported. 
+   <acronym>SSL</acronym> is used interchangeably with <acronym>TLS</acronym>
+   in <productname>PostgreSQL</productname>.
+
+  </para>
   <sect2 id="ssl-setup">
    <title>Basic Setup</title>
 
   <para>
    With <acronym>SSL</acronym> support compiled in, the
    <productname>PostgreSQL</productname> server can be started with
-   <acronym>SSL</acronym> enabled by setting the parameter
+   support for encrypted connections using <acronym>TLS</acronym> protocols
+   enabled by by setting the parameter
    <xref linkend="guc-ssl"/> to <literal>on</literal> in
    <filename>postgresql.conf</filename>.  The server will listen for both normal
    and <acronym>SSL</acronym> connections on the same TCP port, and will negotiate