man: do not say that isolate is like switching runlevels

We need to do better here, but for now let's at least not trick
users into nuking their graphical environment. Inspired by #16548.

TODO

@@ -1087,7 +1087,8 @@ Features:
   - document systemd-journal-flush.service properly
   - documentation: recommend to connect the timer units of a service to the service via Also= in [Install]
   - man: document the very specific env the shutdown drop-in tools live in
-  - man: add more examples to man pages
+  - man: add more examples to man pages,
+  -      in particular an example how to do the equivalent of switching runlevels
   - man: maybe sort directives in man pages, and take sections from --help and apply them to man too
   - document root=gpt-auto properly
 

man/systemctl.xml

@@ -263,11 +263,9 @@ Sun 2017-02-26 20:57:49 EST  2h 3min left  Sun 2017-02-26 11:56:36 EST  6h ago
             If a unit name with no extension is given, an extension of
             <literal>.target</literal> will be assumed.</para>
 
-            <para>This is similar to changing the runlevel in a
-            traditional init system. The <command>isolate</command>
-            command will immediately stop processes that are not enabled
-            in the new unit, possibly including the graphical
-            environment or terminal you are currently using.</para>
+            <para>This command is dangerous, since it will immediately stop processes that are not enabled in
+            the new target, possibly including the graphical environment or terminal you are currently using.
+            </para>
 
             <para>Note that this is allowed only on units where
             <option>AllowIsolate=</option> is enabled. See