bug#8465: <XF86MonBrightnessDown> is undefined

[jidanni]

Why do I get these messages when I hit this key,
<XF86MonBrightnessDown> is undefined
why can't emacs ignore that like bash does?
Just because I adjust the brightness doesn't make it part of emacs'
business. OK, I suppose I need to add something to .emacs to make that
key and its brother no-ops.
P.S., the messages don't end up in the C-h e buffer.
One has to use C-h k to capture them.