help: Document "Jitsi server URL" setting.

- Updates "Start a call", documents the "Jitsi server URL" setting,
  and links to it in a (?) from the relevant admin UI.
- Fixes misplaced include block for opening the compose box.

Fixes #26907.

help/start-a-call.md

@@ -2,10 +2,10 @@
 
 {start_tabs}
 
-{!start-composing.md!}
-
 {tab|desktop-web}
 
+{!start-composing.md!}
+
 1. Click the **video camera** (<i class="fa fa-video-camera"></i>) icon at the
    bottom of the compose box. This will insert a **Join video call.** link into
    your message.
@@ -56,8 +56,8 @@ supported by zulip are:
 
 !!! tip ""
 
-    It is also possible to disable the video call button for your organization by
-    setting the provider to "None".
+    It is also possible to disable the video call button for your organization
+    by setting the provider to "None".
 
 ### Change your organization's video call provider
 
@@ -65,9 +65,29 @@ supported by zulip are:
 
 {settings_tab|organization-settings}
 
-1. Under **Other settings** select appropriate provider from **Call provider** dropdown.
+1. Under **Other settings**, select the desired provider from the
+   **Call provider** dropdown.
 
-1. Click **Save changes**.
+{!save-changes.md!}
+
+{end_tabs}
+
+### Use a self-hosted instance of Jitsi Meet
+
+Zulip uses the [cloud version of Jitsi Meet](https://meet.jit.si/)
+as its default video call provider. You can also use a self-hosted
+instance of Jitsi Meet.
+
+{start_tabs}
+
+{settings_tab|organization-settings}
+
+1. Under **Other settings**, select **Custom URL** from the
+   **Jitsi server URL** dropdown.
+
+1. Enter the URL of your self-hosted Jitsi Meet server.
+
+{!save-changes.md!}
 
 {end_tabs}
 

web/templates/settings/organization_settings_admin.hbs

@@ -108,6 +108,7 @@
                 <div class="input-group">
                     <label for="realm_video_chat_provider" class="dropdown-title">
                         {{t 'Call provider' }}
+                        {{> ../help_link_widget link="/help/start-a-call" }}
                     </label>
                     <select name="realm_video_chat_provider" class ="setting-widget prop-element settings_select bootstrap-focus-style" id="id_realm_video_chat_provider" data-setting-widget-type="number">
                         {{#each realm_available_video_chat_providers}}
@@ -119,6 +120,7 @@
                         <div>
                             <label for="id_realm_jitsi_server_url" class="dropdown-title">
                                 {{t "Jitsi server URL" }}
+                                {{> ../help_link_widget link="/help/start-a-call#configure-a-self-hosted-instance-of-jitsi-meet" }}
                             </label>
                             <select name="realm_jitsi_server_url" id="id_realm_jitsi_server_url" class="setting-widget prop-element settings_select bootstrap-focus-style" data-setting-widget-type="jitsi-server-url-setting">
                                 {{#if server_jitsi_server_url}}