[Bug] Cannot link to constants in Markdown

[UlyssesZh]

How to reproduce:

Create the following files:

# test.rb
module TestModule
  TEST_CONSTANT = 1108
  def test_instance_method
  end
end

<!-- test.md -->
TestModule
TestModule::TEST_CONSTANT
TestModule#test_instance_method

Run the command:

rdoc test.rb test.md

Then, use a browser to open doc/test_md.html.

Result:

Expected:

[st0012]

I can reproduce this but it also happens when renaming .md to .rdoc. So it's likely a general auto-linking issue, not specific markup related.