[Mediawiki-l] Why <top-notice-pagename> page contents <bottom-notice-pagename>

[Monahon, Peter B.]

Okay, reviewing http://www.mediawiki.org/wiki/Extension:PageNotice,
which is responsible for the <top-notice-pagename> page contents
<bottom-notice-pagename>, it says to 

"...define fixed notice messages for the top or the bottom of each page
... by creating MediaWiki messages ..." 

... but I cannot find a reference or example on how to create a system
message! Can anyone be so kind as to direct me to a link showing an
example so that I can implement the extension?  Thanks!