help: Document "Mark messages as read on scroll" mobile app feature.

Adds section on how to configure whether messages are automatically
marked as read.
Adds instructions to manually mark all messages as read, or mark all
messages in a stream or topic as read.
Adds the all messages tab image icon.

Fixes: #22915.

templates/zerver/help/marking-messages-as-read.md

@@ -1,5 +1,21 @@
 # Marking messages as read
 
+## Configure whether messages are automatically marked as read
+
+{start_tabs}
+
+{tab|mobile}
+
+{!mobile-profile-menu.md!}
+
+1. Tap **Settings**.
+
+1. Tap **Mark messages as read on scroll**.
+
+1. Select **Always**, **Never** or **Only in conversation views**.
+
+{end_tabs}
+
 ## Move the blue box to mark messages as read
 In the Zulip desktop or web app, messages are marked as read when the
 **blue box** passes over them. You can move the blue box with the keyboard (see
@@ -29,6 +45,20 @@ stream or topic as read**.
 
 1. Click **Mark all messages as read**.
 
+{tab|mobile}
+
+1. Tap a stream, topic, or the all messages
+   (<img src="/static/images/help/mobile-globe-icon.svg" alt="globe" class="mobile-icon"/>)
+   tab.
+
+1. Tap **Mark stream as read**, **Mark topic as read**, or **Mark all as read**
+   near the top right corner of the app.
+
+!!! tip ""
+
+    You can also press and hold a topic until the long-press menu appears, and
+    select **Mark topic as read**.
+
 {end_tabs}
 
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