docs(autopopulate): fix stale finally-comment; document upstream in class Attributes

Two post-#1499 docstring/comment updates on autopopulate.py:

1. The finally-block comment in _populate_one attributed the 'raises a
   clear error' behavior to `self._upstream = None`, but post-#1499
   that line only invalidates the memoized Diagram. The actual guard
   is `self._upstream_key = None` (via the `upstream` property at
   :135-140). Reworded to attribute each line to its role.

2. AutoPopulate class docstring's Attributes section listed only
   `key_source` and `jobs`, omitting `upstream` - which after #1473 is
   a public property and the recommended read surface inside make().
   Added it alongside the others.

Docs-only.

Author: MilagrosMarin

src/datajoint/autopopulate.py

@@ -89,6 +89,11 @@ class AutoPopulate:
         Query yielding keys to be populated. Default is join of FK parents.
     jobs : Job
         Job table (``~~table_name``) for distributed processing.
+    upstream : Diagram
+        Pre-restricted ancestor view for the current ``make(self, key)`` call
+        (property). Available only inside ``make()``; built lazily on first
+        access and memoized for the call. See the property docstring for
+        details.
 
     Notes
     -----
@@ -736,9 +741,11 @@ def _populate1(
             return True
         finally:
             self.__class__._allow_insert = False
-            # Clear the per-make() upstream view so subsequent attribute
-            # access raises a clear error rather than silently using a
-            # stale trace from the previous make() call.
+            # Clear the per-make() upstream state: `_upstream = None` invalidates
+            # the memoized Diagram; `_upstream_key = None` restores the "outside
+            # make()" state so subsequent `self.upstream` access raises a clear
+            # error via the property guard rather than silently returning a stale
+            # trace from the previous call.
             self._upstream = None
             self._upstream_key = None