docs: describe "unique lines" in details

I wrote a sentence inside "How to sort" to described what happens if
option "output unique lines only" is set.

Signed-off-by: Conrad Zelck <git@simpel.cc>

index.html

@@ -30,6 +30,7 @@
       <summary>How to sort</summary>
       <p>With this tool you can sort newline separated lines alphabetically.</p>
       <p>You can either push the button "Paste, Sort &amp; Copy" where the clipboard content is used as content or fill in your lines into textarea and push the button "Sort &amp; Copy". In both cases the sorted result will be put back to clipboard.</p>
+      <p>If you set the option "output unique lines only" all lines existing more then once are deleted.</p>
     </details>
 <div class="flex">
   <button class="button pastesortunique" title="click: paste + sort + unique + copy to clipboard" onclick="paste()">Paste, Sort &amp; Copy [1]</button>