[DOC] Update missing docs

Author: nobu

gem_prelude.rb

@@ -25,4 +25,3 @@
 rescue LoadError
   warn "`syntax_suggest' was not loaded."
 end if defined?(SyntaxSuggest)
-

lib/bundled_gems.rb

@@ -1,5 +1,10 @@
 # -*- frozen-string-literal: true -*-
 
+# :stopdoc:
+module Gem
+end
+# :startdoc:
+
 module Gem::BUNDLED_GEMS # :nodoc:
   SINCE = {
     "racc" => "3.3.0",

tool/mkconfig.rb

@@ -394,6 +394,7 @@ def RbConfig.ruby
     )
   end
 end
+# Non-nil if configured for cross compiling.
 CROSS_COMPILING = nil unless defined? CROSS_COMPILING
 EOS
 

tool/rdoc-srcdir

@@ -17,7 +17,7 @@ options.title = options.title.sub(/Ruby \K.*version/) {
     .sort # "MAJOR" < "MINOR", fortunately
     .to_h.values.join(".")
 }
-options.parse ARGV
+options.parse ARGV + ["#{invoked}/rbconfig.rb"]
 
 options.singleton_class.define_method(:finish) do
   super()