docs: Add user guide for *mention a team member*.

templates/zerver/help/index.md

@@ -26,6 +26,7 @@ as a “**realm**”.
 
 **[The Zulip browser window](#the-zulip-browser-window)** |
 **[Posting and replying](#posting-and-replying)** |
+**[Mention a Team Member](/help/mention-a-team-member)** |
 **[Other common questions](#other-common-questions)** |
 **[Terminology](#special-terms-used-with-zulip)**
 
@@ -99,13 +100,6 @@ individual users. ![New message](/static/images/help/new_message.png)
    other recipients. For several days, the list of recipients will
    appear under “GROUP PMs” at the lower right corner of your screen.
 
- * You can ping ("**@mention**") other users with a particular
-   stream-message from within the body of the message itself. Type `@`
-   and the beginning of their email address or one of their names, and
-   the system will offer you tab-completions. The completed @mention
-   will look something like this: `@**name@email.address**`, and it
-   will appear as **@name** in your posted message.
-
  * If you’re bashful about using the pronoun “I”, you can get your own
    registered name to appear boldfaced in a message by entering
    `/me`. At present it has to be the first thing on a line, and

templates/zerver/help/mention-a-team-member.md

@@ -0,0 +1,11 @@
+#Mention a Team Member
+
+![Mention a Team Member](/static/images/help/mention_user.png)
+
+When having a conversation in Zulip, it is sometimes necessary to call the attention of another member. You can alert ("**@mention**") other users with a particular stream-message from within the body of the message itself. Type `@` and then begin typing their email address or one of their names, and the system will offer you tab-completions. The completed `@mention` will look something like: `@**name**`, and it will appear as **@name** in your posted message.
+
+If you attempt to `@mention` someone who is not subscribed to the current stream, Zulip will ask if you would like to subscribe them to the stream:
+
+![A User Who is Not Subscribed](/static/images/help/mention_user_not_subscribed.png)
+
+You can view any mentions calling your attention by clicking on the "@-mentions" view in the left sidebar.

zerver/tests/test_templates.py

@@ -98,6 +98,7 @@ class TemplateTestCase(ZulipTestCase):
             'zerver/zulipchat_migration_tos.html',
             'zilencer/enterprise_tos_accept_body.txt',
             'zerver/help/index.md',
+            'zerver/help/mention-a-team-member.md',
             'zerver/help/missing.md',
             'zerver/closed_realm.html',
             'zerver/topic_is_muted.html',