docs: Add some more links to new soft deactivation documentation.

zerver/lib/actions.py

@@ -1488,6 +1488,9 @@ def create_user_messages(message: Message,
     #   case the notifications code will call `access_message` on the
     #   message to re-verify permissions, and for private streams,
     #   will get an error if the UserMessage row doesn't exist yet.
+    #
+    # See https://zulip.readthedocs.io/en/latest/subsystems/sending-messages.html#soft-deactivation
+    # for details on this system.
     user_messages = []
     for um in ums_to_create:
         if (um.user_profile_id in long_term_idle_user_ids and

zerver/lib/soft_deactivation.py

@@ -1,3 +1,4 @@
+# Documented in https://zulip.readthedocs.io/en/latest/subsystems/sending-messages.html#soft-deactivation
 
 from zerver.lib.logging_util import log_to_file
 from collections import defaultdict
@@ -96,6 +97,10 @@ def add_missing_messages(user_profile: UserProfile) -> None:
 
     * Create the UserMessage rows.
 
+    For further documentation, see:
+
+      https://zulip.readthedocs.io/en/latest/subsystems/sending-messages.html#soft-deactivation
+
     """
     assert user_profile.last_active_message_id is not None
     all_stream_subs = list(Subscription.objects.select_related('recipient').filter(