Since the number of monitors can change at any time, reliable
use of these functions requires handling failure in some way.
Handling #f results is easier (and less likely to mask other
problems) than catching exceptions.
I originally picked "under" as the preposition to go before
a platform name, but obviously you should build "on" a
platform, and "under" suddenly annoys me. The choice of "on"
is now codified in the documentation style guide. Meanwhile,
"Unix" insted of "X" seems more clear and consistent in the
`racket/gui' docs.
More usefully, this patch also fixes a few out-of-date
platform-specific claims.
