rdoc update.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@22607 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

file.c

@@ -3499,15 +3499,18 @@ rb_thread_flock(void *data)
  *
  *  Example:
  *
- *     # write lock
+ *     # update a counter using write lock
  *     # don't use "w" because it truncates the file before lock.
- *     File.open("testfile", File::WRONLY|File::CREAT, 0644) {|f|
+ *     File.open("counter", File::RDWR|File::CREAT, 0644) {|f|
  *       f.flock(File::LOCK_EX)
- *       f.truncate(0)
- *       f.write "new content"
+ *       value = f.read.to_i + 1
+ *       f.rewind
+ *       f.write("#{value}\n")
+ *       f.flush
+ *       f.truncate(f.pos)
  *     }
  *
- *     # read lock
+ *     # read the counter using read lock
  *     File.open("testfile", "r") {|f|
  *       f.flock(File::LOCK_SH)
  *       p f.read