os: rearrange OpenFile Flags doc

Updates #21322

Change-Id: Ib03ee9dbe1b44c2fecd51f2f2c23a88482158e7e
Reviewed-on: https://go-review.googlesource.com/75250Reviewed-by: Rob Pike <r@golang.org>

src/os/file.go

@@ -60,12 +60,13 @@ var (
 )
 
 // Flags to OpenFile wrapping those of the underlying system. Not all
-// flags may be implemented on a given system. Each call to OpenFile
-// should specify exactly one of O_RDONLY, O_WRONLY, or O_RDWR.
+// flags may be implemented on a given system.
 const (
+	// Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
 	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
 	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
 	O_RDWR   int = syscall.O_RDWR   // open the file read-write.
+	// The remaining values may be or'ed in to control behavior.
 	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
 	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
 	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.