docs: fix screenshot caption overclaiming read states

The chat screenshot shows messages + typing; "read states" reads as read
receipts (which don't exist). Backspace tracks unread state only. Reworded
captions to replies/reactions/typing.

README.md

@@ -44,7 +44,7 @@ inside a business — but not to resell as a hosted service. See
   <tr>
     <td width="50%" valign="top">
       <img src="docs/screenshots/chat.webp" alt="A text channel with messages and a typing indicator" /><br/>
-      <sub><b>Text channels</b> — Markdown, replies, reactions, read states, and live typing indicators.</sub>
+      <sub><b>Text channels</b> — Markdown, replies, reactions, and live typing indicators.</sub>
     </td>
     <td width="50%" valign="top">
       <img src="docs/screenshots/screen-share-settings.webp" alt="The screen-share settings popover" /><br/>

docs/screenshots.md

@@ -33,7 +33,7 @@ and live screen-shares, each labelled with its own resolution and frame rate.
 
 <div align="center">
   <img src="screenshots/chat.webp" alt="A text channel with an active conversation and a typing indicator" width="900" /><br/>
-  <sub>Real-time messages with read states and live typing indicators.</sub>
+  <sub>Real-time messages with replies, reactions, and live typing indicators.</sub>
 </div>
 
 <br/>