doc: surface the sign-in and send hint

ChatGPT can require signing in, and it does not always send a prefilled
prompt. The README and options panel mentioned pressing Enter only in
passing. Give both a prominent note covering sign-in and Enter or Send,
and drop the inline mentions it replaces.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

options/options.html

@@ -9,8 +9,7 @@
 <p class="intro">
   This extension adds <a href="https://chatgpt.com/" target="_blank" rel="noopener noreferrer">ChatGPT</a> as a Firefox search engine. Type
   <code>gpt your question</code> in the address bar, select text on a webpage and
-  choose <strong>Ask ChatGPT</strong> from the right-click menu.<span id="shortcut-intro"></span> If the prompt isn’t sent automatically, press Enter in
-  ChatGPT.
+  choose <strong>Ask ChatGPT</strong> from the right-click menu.<span id="shortcut-intro"></span>
 </p>
 
 <p class="intro">
@@ -19,6 +18,11 @@
   every context-menu mode and follows your settings below.
 </p>
 
+<p class="intro">
+  ChatGPT may require you to sign in. If your prompt is not sent automatically,
+  press <strong>Enter</strong> or click <strong>Send</strong> in ChatGPT.
+</p>
+
 <form id="settings" autocomplete="off">
   <div class="field">
     <label for="contextMenuMode">Context menu</label>