Update docs

_sources/usage.md.txt

@@ -204,6 +204,9 @@ The full set of configuration options are:
     (delegated) permission to the app. If you are using
     `UsernamePassword` auth and the mailbox is different from the
     username, you must grant the app `Mail.ReadWrite.Shared`.
+
+    The app registration must have the `Treat application as public client`
+    option enabled.
     :::
 
     :::{warning}

usage.html

@@ -304,6 +304,8 @@ admin grant the Microsoft Graph <code class="docutils literal notranslate"><span
 (delegated) permission to the app. If you are using
 <code class="docutils literal notranslate"><span class="pre">UsernamePassword</span></code> auth and the mailbox is different from the
 username, you must grant the app <code class="docutils literal notranslate"><span class="pre">Mail.ReadWrite.Shared</span></code>.</p>
+<p>The app registration must have the <code class="docutils literal notranslate"><span class="pre">Treat</span> <span class="pre">application</span> <span class="pre">as</span> <span class="pre">public</span> <span class="pre">client</span></code>
+option enabled.</p>
 </div>
 <div class="admonition warning">
 <p class="admonition-title">Warning</p>