Merge RDoc 6.0.3 from upstream.

It fixed the several bugs that was found after RDoc 6 releasing.

From: SHIBATA Hiroshi <hsbt@ruby-lang.org>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rdoc/test_rdoc_text.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
-require 'rdoc/test_case'
+require 'minitest_helper'
+require 'timeout'
 
 class TestRDocText < RDoc::TestCase
 
@@ -377,6 +378,32 @@ paragraph will be cut off …
     assert_equal expected, strip_stars(text)
   end
 
+  def test_strip_stars_document_method_special
+    text = <<-TEXT
+/*
+ * Document-method: Zlib::GzipFile#mtime=
+ * Document-method: []
+ * Document-method: `
+ * Document-method: |
+ * Document-method: &
+ * Document-method: <=>
+ * Document-method: =~
+ * Document-method: +
+ * Document-method: -
+ * Document-method: +@
+ *
+ * A comment
+ */
+    TEXT
+
+    expected = <<-EXPECTED
+
+   A comment
+    EXPECTED
+
+    assert_equal expected, strip_stars(text)
+  end
+
   def test_strip_stars_encoding
     text = <<-TEXT
 /*