Don't document aliases with trailing :nodoc directive

Attribute readers and writers can be marked as `:nodoc` to keep them
undocumented:

```ruby
attr_reader :name # :nodoc:
```

For aliases this behaviour should be the same:

```ruby
alias_method :old :new # :nodoc:
```

lib/rdoc/parser/ruby.rb

@@ -789,8 +789,10 @@ def parse_alias(context, single, tk, comment)
     al.line   = line_no
 
     read_documentation_modifiers al, RDoc::ATTR_MODIFIERS
-    context.add_alias al
-    @stats.add_alias al
+    if al.document_self or not @track_visibility
+      context.add_alias al
+      @stats.add_alias al
+    end
 
     al
   end

test/rdoc/test_rdoc_parser_ruby.rb

@@ -3065,6 +3065,28 @@ def test_parse_statements_identifier_yields
     assert_nil m.params, 'Module parameter not removed'
   end
 
+  def test_parse_statements_nodoc_identifier_alias
+    klass = @top_level.add_class RDoc::NormalClass, 'Foo'
+
+    util_parser "\nalias :old :new # :nodoc:"
+
+    @parser.parse_statements klass, RDoc::Parser::Ruby::NORMAL, nil
+
+    assert_empty klass.aliases
+    assert_empty klass.unmatched_alias_lists
+  end
+
+  def test_parse_statements_nodoc_identifier_alias_method
+    klass = @top_level.add_class RDoc::NormalClass, 'Foo'
+
+    util_parser "\nalias_method :old :new # :nodoc:"
+
+    @parser.parse_statements klass, RDoc::Parser::Ruby::NORMAL, nil
+
+    assert_empty klass.aliases
+    assert_empty klass.unmatched_alias_lists
+  end
+
   def test_parse_statements_stopdoc_alias
     klass = @top_level.add_class RDoc::NormalClass, 'Foo'