Fix LoadError's linking issue

Original issue: https://github.com/ruby/rdoc/issues/1128

The problem is caused by the `# :stopdoc:` directive in `bundled_gems.rb`,
which's scope covers the redefinition of `LoadError`.

Since the goal of `# :stopdoc:` is to hide the documentation of `Gem::BUNDLED_GEMS`,
we can use `# :nodoc:` on it instead.

error.c

@@ -3401,7 +3401,7 @@ syserr_eqq(VALUE self, VALUE exc)
  *
  *  * NoMemoryError
  *  * ScriptError
- *    * {LoadError}[https://docs.ruby-lang.org/en/master/LoadError.html]
+ *    * LoadError
  *    * NotImplementedError
  *    * SyntaxError
  *  * SecurityError

lib/bundled_gems.rb

@@ -1,8 +1,6 @@
 # -*- frozen-string-literal: true -*-
 
-# :stopdoc:
-
-module Gem::BUNDLED_GEMS
+module Gem::BUNDLED_GEMS # :nodoc:
   SINCE = {
     "matrix" => "3.1.0",
     "net-ftp" => "3.1.0",
@@ -247,7 +245,7 @@ end
 # for RubyGems without Bundler environment.
 # If loading library is not part of the default gems and the bundled gems, warn it.
 class LoadError
-  def message
+  def message # :nodoc:
     return super unless path
 
     name = path.tr("/", "-")
@@ -257,5 +255,3 @@ class LoadError
     super
   end
 end
-
-# :startdoc: