user docs: Add user guide for *View a message's edit history* feature.

Fixes #3730

templates/zerver/help/index.md

@@ -64,6 +64,7 @@ as an **organization**.
 ### Editing
 * [Edit or delete a message](/help/edit-or-delete-a-message)
 * [Change the topic of a message](/help/change-the-topic-of-a-message)
+* [View a message's edit history](/help/view-a-messages-edit-history)
 
 ## People
 * [Check whether someone is here or away](/help/check-whether-someone-is-here-or-away)

templates/zerver/help/view-a-messages-edit-history.md

@@ -0,0 +1,32 @@
+# View a message's edit history
+
+You can view the edit history of any edited messages in your organization.
+
+{!message-actions.md!}
+{!down-chevron.md!}
+
+3. Click on the **View edit history** option from the actions dropdown.
+
+4. Upon clicking on the **View edit history** option, the **Message edit history**
+modal will appear.
+
+## Message edit history
+
+![Message edit history modal](/static/images/help/message-edit-history.png)
+
+The **Message edit history** modal shows all edits to a message, sorted from
+earliest to latest.
+
+Each entry in the **Message edit history** modal has the following properties:
+
+1. Highlighted **differences** (additions in green, deletions in red) between
+the previous message edit and the current version.
+
+2. **Timestamp** of the message edit.
+
+3. **Author** of the message edit.
+
+!!! tip ""
+    To exit the **Message edit history** modal, simply click the x (<i
+    class="icon-vector-remove"></i>) icon in the top right-hand corner or the
+    **Close** button in the bottom right corner.