[ruby/io-console] Use :nodoc: instead of :stopdoc: for internal

C-defined classes
(ruby/io-console#123)

RDoc does not officially support :stopdoc:/:startdoc: in C files,
and recent RDoc changes cause :stopdoc: to be incorrectly undone
during file processing, making these classes appear as undocumented
in coverage reports.

Switch to :nodoc: which is properly supported in C comments and
correctly marks these internal classes as intentionally undocumented.

ruby/io-console@451630c6ce

Author: st0012

ext/io/console/console.c

@@ -2000,14 +2000,13 @@ InitVM_console(void)
     rb_define_method(rb_cIO, "ttyname", console_ttyname, 0);
     rb_define_singleton_method(rb_cIO, "console", console_dev, -1);
     {
-	/* :stopdoc: */
+	/* :nodoc: */
 	VALUE mReadable = rb_define_module_under(rb_cIO, "generic_readable");
-	/* :startdoc: */
 	rb_define_method(mReadable, "getch", io_getch, -1);
 	rb_define_method(mReadable, "getpass", io_getpass, -1);
     }
     {
-	/* :stopdoc: */
+	/* :nodoc: */
         cConmode = rb_define_class_under(rb_cIO, "ConsoleMode", rb_cObject);
         rb_define_const(cConmode, "VERSION", rb_obj_freeze(rb_str_new_cstr(IO_CONSOLE_VERSION)));
         rb_define_alloc_func(cConmode, conmode_alloc);
@@ -2016,6 +2015,5 @@ InitVM_console(void)
         rb_define_method(cConmode, "echo=", conmode_set_echo, 1);
         rb_define_method(cConmode, "raw!", conmode_set_raw, -1);
         rb_define_method(cConmode, "raw", conmode_raw_new, -1);
-	/* :startdoc: */
     }
 }