app: [API] remove Main

All platforms already allow the omission of the call to Main and running
Windows on the main goroutine. This change just gets rid of Main, and
documents the special requirement on Window.Event.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/doc.go

@@ -33,28 +33,12 @@ For example:
 A program must keep receiving events from the event channel until
 [DestroyEvent] is received.
 
-# Main
+# Main Thread
 
-The Main function must be called from a program's main function, to hand over
-control of the main thread to operating systems that need it.
-
-Because Main is also blocking on some platforms, the event loop of a Window must run in a goroutine.
-
-For example, to display a blank but otherwise functional window:
-
-	package main
-
-	import "gioui.org/app"
-
-	func main() {
-		go func() {
-			w := app.NewWindow()
-			for {
-				w.Event()
-			}
-		}()
-		app.Main()
-	}
+Some GUI platform need access to the main thread of the program. To avoid a
+deadlock on such platforms, at least one Window must have its Event method
+called by the main goroutine. It doesn't have to be any particular Window;
+even a destroyed Window suffices.
 
 # Permissions