doc: Add acronyms for MITM and SNI

This adds MITM and SNI as acronyms, as the documentation already had
them marked up with <acronym>.

While on it, make sure to spell man-in-the-middle with dashes
consistently, and add acronyms for those new terms where appropriate.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CE12DD5C-4BB3-4166-BC9A-39779568734C@yesql.se

doc/src/sgml/acronyms.sgml

@@ -410,6 +410,17 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><acronym>MITM</acronym></term>
+    <listitem>
+     <para>
+      <ulink
+      url="https://en.wikipedia.org/wiki/Man-in-the-middle_attack">
+      Man-in-the-middle attack</ulink>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>MSVC</acronym></term>
     <listitem>
@@ -590,6 +601,18 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><acronym>SNI</acronym></term>
+    <listitem>
+     <para>
+      <ulink
+      url="https://en.wikipedia.org/wiki/Server_Name_Indication">
+       Server Name Indication</ulink>,
+      <ulink url="https://tools.ietf.org/html/rfc6066#section-3">RFC 6066</ulink>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>SPI</acronym></term>
     <listitem>

doc/src/sgml/config.sgml

@@ -1378,7 +1378,7 @@ include_dir 'conf.d'
           <listitem>
            <para>
             Disables anonymous cipher suites that do no authentication.  Such
-            cipher suites are vulnerable to man-in-the-middle attacks and
+            cipher suites are vulnerable to <acronym>MITM</acronym> attacks and
             therefore should not be used.
            </para>
           </listitem>

doc/src/sgml/libpq.sgml

@@ -1783,18 +1783,17 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
       <listitem>
        <para>
         By default, libpq sets the TLS extension <quote>Server Name
-        Indication</quote> (SNI) on SSL-enabled connections.  See <ulink
-        url="https://tools.ietf.org/html/rfc6066#section-3">RFC 6066</ulink>
-        for details.  By setting this parameter to 0, this is turned off.
+        Indication</quote> (<acronym>SNI</acronym>) on SSL-enabled connections.
+        By setting this parameter to 0, this is turned off.
        </para>
 
        <para>
         The Server Name Indication can be used by SSL-aware proxies to route
         connections without having to decrypt the SSL stream.  (Note that this
         requires a proxy that is aware of the PostgreSQL protocol handshake,
-        not just any SSL proxy.)  However, SNI makes the destination host name
-        appear in cleartext in the network traffic, so it might be undesirable
-        in some cases.
+        not just any SSL proxy.)  However, <acronym>SNI</acronym> makes the
+        destination host name appear in cleartext in the network traffic, so
+        it might be undesirable in some cases.
        </para>
       </listitem>
      </varlistentry>
@@ -8430,7 +8429,7 @@ ldap://ldap.acme.com/cn=dbserver,cn=hosts?pgconnectinfo?base?(objectclass=*)
     </varlistentry>
 
     <varlistentry>
-     <term>Man in the middle (<acronym>MITM</acronym>)</term>
+     <term>Man-in-the-middle (<acronym>MITM</acronym>)</term>
      <listitem>
       <para>If a third party can modify the data while passing between the
        client and server, it can pretend to be the server and therefore see and