refine alpha build description and add compatibility note

Author: teetow

src/pages/au4.astro

@@ -18,8 +18,8 @@ const { win, mac, lin } = betaReleases;
         <h1>Audacity&nbsp;4 Alpha 1</h1>
         <div class="mt-4">
           <p>
-            The purpose of this alpha build is to test core aspects of Audacity
-            4. You can help us refine the experience by chiming in on the
+            The purpose of this alpha build is to test core features of Audacity
+            4. You can help us refine the user experience by chiming in on the
             forums. If you experience severe issues or crashes, please file an
             issue on GitHub.
           </p>
@@ -39,7 +39,7 @@ const { win, mac, lin } = betaReleases;
             </li>
           </ul>
 
-          <h2 class="mt-6 text-2xl">What's in the build</h2>
+          <h2 class="mt-6 text-2xl">What's implemented</h2>
 
           <ul class="list-disc mt-2 ml-6">
             <li>Recording and playback</li>
@@ -52,6 +52,13 @@ const { win, mac, lin } = betaReleases;
             </li>
           </ul>
 
+          <h3 class="mt-2">A note on compatibility ⚠️</h3>
+          <p class="mt-2">
+            Projects saved in Audacity 4 are not backwards compatible. We
+            recommend you make a copy of your important projects before opening
+            them in Audacity 4.
+          </p>
+
           <h2 class="mt-6 text-2xl">Unimplemented features</h2>
           <p class="mt-2">
             Some yet-to-be finalized features might appear non-functional, or