doc: clarify limitation of parsing values in "NetworkManager.conf"

This really should be fixed, because it's a non-obvious limitation.
Anyway, document it for now.

man/NetworkManager.conf.xml

@@ -823,6 +823,11 @@ domains=ALL
       and these default values only matter if the per-profile values explicitly indicates
       to use the default from <literal>NetworkManager.conf</literal>.
     </para>
+    <para>
+      Note that while nmcli supports various aliases and convenience features for configuring
+      properties, the settings in this section do not. For example, enum values usually only
+      can be configured via their numeric magic number.
+    </para>
     <para>
       Example:
 <programlisting>