[ruby/stringio] [DOC] Tweaks for StringIO#close

(ruby/stringio#148)

Make examples do more work (instead of text).

ruby/stringio@7f4662438f

Author: BurdetteLamar

ext/stringio/stringio.c

@@ -549,11 +549,16 @@ strio_set_string(VALUE self, VALUE string)
  * call-seq:
  *   close -> nil
  *
- * Closes +self+ for both reading and writing.
+ * Closes +self+ for both reading and writing; returns +nil+:
  *
- * Raises IOError if reading or writing is attempted.
+ *   strio = StringIO.new
+ *   strio.closed? # => false
+ *   strio.close   # => nil
+ *   strio.closed? # => true
+ *   strio.read    # Raises IOError: not opened for reading
+ *   strio.write   # Raises IOError: not opened for writing
  *
- * Related: StringIO#close_read, StringIO#close_write.
+ * Related: StringIO#close_read, StringIO#close_write, StringIO.closed?.
  */
 static VALUE
 strio_close(VALUE self)