When performing a search on the Codex, you’re presented with a slew of search results. However, not all of those results are within the Codex. The search portion of the Codex is powered by a Google custom search box which not only presents results from within the Codex, but from across WordPress.org as well, mainly the support forum.
While performing a search for Conditional Statements, the first result was the one I was looking for. However, if you want the results to strictly be within the Codex, I came across this link shared by Otto on the Documentation mailing list. While giving this method of searching the Codex a try, I found it difficult to find the Conditional Statements page I was looking for that was easily displayed by the Google Custom Search box. Even by checking each box, I failed at finding the page using both Conditional Statements and Conditional as my search terms.
My advice, stick to using the Search box that exists on the Codex page.