man: note for systemctl clean --what that commas separate values

(cherry picked from commit 652e4dd743)

man/systemctl.xml

@@ -573,7 +573,8 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
             command only applies to units that use either of these settings. If <option>--what=</option> is
             not specified, the cache and runtime data as well as the file descriptor store are removed (as
             these three types of resources are generally redundant and reproducible on the next invocation of
-            the unit). Note that the specified units must be stopped to invoke this operation.</para>
+            the unit). Multiple values can be seperated by commas. Note that the specified units must be stopped
+            to invoke this operation.</para>
 
             <table>
               <title>