presence: Document that a user can disable their presence updates.

This is a crucial detail, as it makes UserPresence an unreliable
indicator of when users were recently active. It should be documented
more clearly.

docs/subsystems/presence.md

@@ -45,6 +45,9 @@ about that data structure:
   accurately compute whether that user is offline (even if the last
   data from the server was 45 seconds ago, and the user was last
   online 4:30 before the client received that server data).
+* Users can disable their own presence updates in user settings
+  (`UserProfile.presence_enabled` is the flag storing [this user
+  preference](https://zulip.com/help/status-and-availability#disable-updating-availability)).
 * The `status_from_timestamp` function in `static/js/presence.js` is
   useful sample code; the `OFFLINE_THRESHOLD_SECS` check is critical
   to correct output.

zerver/models.py

@@ -2817,6 +2817,17 @@ def get_huddle_backend(huddle_hash: str, id_list: List[int]) -> Huddle:
 
 
 class UserActivity(models.Model):
+    """Data table recording the last time each user hit Zulip endpoints
+    via which Clients; unlike UserPresence, these data are not exposed
+    to users via the Zulip API.
+
+    Useful for debugging as well as to answer analytics questions like
+    "How many users have accessed the Zulip mobile app in the last
+    month?" or "Which users/organizations have recently used API
+    endpoint X that is about to be desupported" for communications
+    and database migration purposes.
+    """
+
     id: int = models.AutoField(auto_created=True, primary_key=True, verbose_name="ID")
     user_profile: UserProfile = models.ForeignKey(UserProfile, on_delete=CASCADE)
     client: Client = models.ForeignKey(Client, on_delete=CASCADE)
@@ -2841,6 +2852,10 @@ class UserActivityInterval(models.Model):
 class UserPresence(models.Model):
     """A record from the last time we heard from a given user on a given client.
 
+    NOTE: Users can disable updates to this table (see UserProfile.presence_enabled),
+    so this cannot be used to determine if a user was recently active on Zulip.
+    The UserActivity table is recommended for that purpose.
+
     This is a tricky subsystem, because it is highly optimized.  See the docs:
       https://zulip.readthedocs.io/en/latest/subsystems/presence.html
     """