man: rework resolved.conf's Cache= documentation

Let's not mention the supposed security benefit of turning off caching. It is
really questionnable, and I#d rather not create the impression that we actually
believed turning off caching would be a good idea.

Instead, mention that Cache=no is implicit if a DNS server on the local host is
used.

man/resolved.conf.xml

@@ -204,19 +204,13 @@
 
       <varlistentry>
         <term><varname>Cache=</varname></term>
-        <listitem><para>Takes a boolean argument. If "yes" (the default),
-        resolving a domain name which already got queried earlier will re-use
-        the previous result as long as that is still valid, and thus does not
-        need to do an actual network request.</para>
+        <listitem><para>Takes a boolean argument. If "yes" (the default), resolving a domain name which already got
+        queried earlier will return the previous result as long as it is still valid, and thus does not result in a new
+        network request. Be aware that that turning off caching comes at a performance penalty, which is particularly
+        high when DNSSEC is used.</para>
 
-        <para>However, local caching slightly increases the chance of a
-        successful DNS poisoning attack, and might also be a privacy problem in
-        some environments: By measuring the time it takes to resolve a
-        particular network name, a user can determine whether any other user on
-        the same machine recently visited that name. If either of these is a
-        concern, you may disable the local caching. Be aware that this comes at
-        a performance cost, which is <emphasis>very</emphasis> high with DNSSEC.
-        </para></listitem>
+        <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
+        (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
       </varlistentry>
 
     </variablelist>