Clarify the /etc/udev/hwdb.d file override with respect to UDEV_HWDB_PATH

Overriding a file in /etc/udev/hwdb.d also hides the files with that
name in UDEV_HWDB_PATH.

Author: Vivien Kraus

man/udev.xml

@@ -748,7 +748,8 @@
       take precedence over files with the same name and lower priority.
       This order can be used to override a system-supplied hwdb file with a
       local file if needed; a symlink in <filename>/etc</filename> with the
-      same name as a hwdb file in <filename>/usr/lib</filename>, pointing to
+      same name as a hwdb file in <filename>/usr/lib</filename> or
+      <envar>UDEV_HWDB_PATH</envar>, pointing to
       <filename>/dev/null</filename>, disables the hwdb file entirely. hwdb
       files must have the extension <filename>.hwdb</filename>; other
       extensions are ignored.</para>