help: Add an article about moving content to another topic.

This commit addresses @timabbott 's comments, by adding a new article
"Move content to another topic".

Thanks to the new article, I also simplified a step from the "Rename a
topic" article to keep it focused on renaming only.

templates/zerver/help/include/sidebar_index.md

@@ -71,6 +71,7 @@
 * [Start a new topic](/help/start-a-new-topic)
 * [Rename a topic](/help/rename-a-topic)
 * [Resolve a topic](/help/resolve-a-topic)
+* [Move content to another topic](/help/move-content-to-another-topic)
 * [Manage inactive streams](/help/manage-inactive-streams)
 
 ## Notifications

templates/zerver/help/move-content-to-another-topic.md

@@ -0,0 +1,37 @@
+# Move content to another topic
+
+By default, any user can change the topic of any message. This is useful for
+keeping messages organized even if some users are still learning how to use
+topics effectively.
+
+Organization administrators can
+[configure who can edit topics](/help/configure-who-can-edit-topics), or turn off
+message editing entirely. See the
+[guide to message and topic editing](/help/configure-message-editing-and-deletion)
+for the details on when topic editing is allowed.
+
+### Move a message to another topic
+
+{start_tabs}
+
+{!message-actions-menu.md!}
+
+1. Select the first option. It may be called **View source / Edit topic**,
+   or simply **Edit**. If it's called **View source**, then you are not
+   allowed to edit the topic of that message.
+
+2. Set the destination topic.
+
+3. A dropdown with three options will appear to the right:
+**Change only this message topic**, **Change later messages to this topic**, and
+**Change previous and following messages to this topic**. Pick the appropriate
+option.
+
+4. Click **Save**.
+
+{end_tabs}
+
+## Related articles
+
+* [Rename a topic](/help/rename-a-topic)
+* [Move content to another stream](/help/move-content-to-another-stream)

templates/zerver/help/rename-a-topic.md

@@ -29,10 +29,8 @@ for the details on when topic editing is allowed.
 
 1. Edit the topic.
 
-1. A dropdown with three options will appear to the right:
-**Change only this message topic**, **Change later messages to this topic**, and
-**Change previous and following messages to this topic**. Pick the appropriate
-option.
+1. Pick **Change previous and following messages to this topic** from the
+   dropdown to the right.
 
 1. Click **Save**.