title: Can't use :ref: with a Label using Sphinx doc
body: <p>I have a problem with using Sphinx-doc's <code>:ref:</code> role, I put a label above a paragraph and then I try to link to that label from another doc but inside the same project.</p>
  
  <p>The label I use in one document:</p>
  
  <pre><code>.. _hal_1k_1p:
  </code></pre>
  
  <p>And the <code>ref</code> I try to use to link to that label:</p>
  
  <pre><code>:ref:`Link title hal_1k_1p`
  </code></pre>
  
  <p>But I only get this error message when building the doc:</p>
  
  <blockquote>
    <p>WARNING: undefined label: link title hal_1k_1p (if the link has no
    caption the label must precede a section header)</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>