The current text is a bit terse:

"Here you can set HTML text which will display as a banner in all JIRA pages. This is useful for alerting users of upcoming system-wide changes.

and

This field accepts html, be sure to close all tags"

Something such as this might be useful:

"
You can set HTML text which will display as a banner at the top of all pages in JIRA. This is useful for alerting users of upcoming system-wide changes.

and

This field accepts HTML, be sure to close all tags."
"

And then add some precanned messages including ones with nice Confluence box macros as described at

http://confluence.atlassian.com/display/JIRACOM/Announcement+Banner+templates?focusedCommentId=189431898&#comment-189431898

and offer them as choices in the same way that the different services are presented in the Admin, Services screen.

~Matt