man: describe masking of .network files better

This should clear up some confusion in
https://github.com/systemd/systemd/issues/717.

This basically copies the description from systemd.unit to this
man page. Masking can happen also in /run, so strike the part
about /etc, and also add the magic work "mask".

man/systemd.netdev.xml

@@ -80,11 +80,9 @@
     <filename>/run</filename> take precedence over files with the same
     name in <filename>/usr/lib</filename>. This can be used to
     override a system-supplied configuration file with a local file if
-    needed; a symlink in <filename>/etc</filename> with the same name
-    as a configuration file in <filename>/usr/lib</filename>, pointing
-    to <filename>/dev/null</filename>, disables the configuration file
-    entirely.</para>
-
+    needed. As a special case, an empty file (file size 0) or symlink
+    with the same name pointing to <filename>/dev/null</filename>,
+    disable the configuration file entirely (it is "masked").</para>
   </refsect1>
 
   <refsect1>