"under <platform>" -> "on <platform>"; "X" -> "Unix"

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 More usefully, this patch also fixes a few out-of-date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.

original commit: f34a31cac945a622ea5994ca270cc5e253c795ed

collects/scribblings/scribble/style.scrbl

@@ -58,6 +58,15 @@ than a hidden abstraction in the document implementation.
 
 Hyphenate the words ``sub-form'' and ``sub-expression.''
 
+Use ``Windows,'' ``Mac OS X,'' and ``Unix'' for the three
+``platforms'' (as opposed to ``systems'') on which Racket runs. Use
+``Unix'' as a generic term for Unix-like operating systems---notably
+including Linux---other than Mac OS X. Use ``Unix'' even when ``Gtk''
+or ``the X11 windowing system'' would be more precisely correct, but
+use ``X11'' as adjective when necessary, such as ``X11 display.''
+Racket runs ``on'' a platform, as opposed to ``under'' a platform.
+
+
 @section{Typesetting Code}
 
 Use @racketidfont{id} or a name that ends @racketidfont{-id} in