help-docs: Note automated PMs sent when subscribing users to streams.

Adds a shared note to both help center articles with sections on
subscribing users to streams, documenting that an automated private
message from the notification bot is sent to a user when subscribed
to a stream by another user.

Links to the article on the notification bot, and clarifies text in
that article regarding automated private messages.

templates/zerver/help/add-or-remove-users-from-a-stream.md

@@ -31,6 +31,8 @@ to a stream][configure-invites].
       To add users in bulk, you can copy members from an
       existing stream or [user group](/help/user-groups).
 
+{!automated-pm-stream-subscription.md!}
+
 ### Mentioning a user in the compose box (alternate method)
 
 When you [mention a user](/help/mention-a-user-or-group) while composing

templates/zerver/help/configure-notification-bot.md

@@ -12,9 +12,9 @@ various organization level events, including:
 * New public stream announcements (private streams are not announced)
 * New user announcements
 
-The notification bot also generates messages to individual users
-for some user specific events, such as [being subscribed to a
-stream][add-users-to-stream] by another user.
+The notification bot also generates automated private messages to
+individual users for some user specific events, such as [being
+subscribed to a stream][add-users-to-stream] by another user.
 
 Organization administrators can configure where (and whether)
 [new stream](#new-stream-announcements) and
@@ -23,8 +23,8 @@ Organization administrators can configure where (and whether)
 Stream messages sent by the notification bot (including the topic)
 are translated into the language that the organization has configured
 as the [organization language for automated messages and invitation
-emails][org-lang]. Notification bot messages sent to a single user will
-use [their preferred language](/help/change-your-language).
+emails][org-lang]. Private messages sent by the notification bot to
+a user will use [their preferred language](/help/change-your-language).
 
 ## Configure notification bot
 

templates/zerver/help/include/automated-pm-stream-subscription.md

@@ -0,0 +1,6 @@
+!!! warn ""
+
+      **Note**: Subscribing someone else to a stream sends them an
+      automated private message from [notification bot][notification-bot].
+
+[notification-bot]: /help/configure-notification-bot

templates/zerver/help/manage-user-stream-subscriptions.md

@@ -4,7 +4,7 @@
 
 !!! warn ""
 
-      Note that the list of a user's **Subscribed streams** will be limited to
+      **Note**: The list of a user's **Subscribed streams** will be limited to
       streams for which [you can see all subscribers](/help/stream-permissions).
 
 {start_tabs}
@@ -35,6 +35,8 @@ other users to a stream][configure-invites].
 
 {end_tabs}
 
+{!automated-pm-stream-subscription.md!}
+
 ## Unsubscribe a user from stream(s)
 
 {!admin-only.md!}