doc: Define TLS as an acronym

Commit c6763156 added an acronym reference for "TLS" but the definition
was never added.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier
Backpatch-through: 9.6
Discussion: https://postgr.es/m/27109504-82DB-41A8-8E63-C0498314F5B0@yesql.se

doc/src/sgml/acronyms.sgml

@@ -684,6 +684,16 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><acronym>TLS</acronym></term>
+    <listitem>
+     <para>
+      <ulink url="https://en.wikipedia.org/wiki/Transport_Layer_Security">
+      Transport Layer Security</ulink>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>TOAST</acronym></term>
     <listitem>