[3.1.x] Fixed #32388 -- Clarified QuerySet.bulk_update() caveat about…

… duplicates for multiple batches.

Backport of 241da3f from master

docs/ref/models/querysets.txt

@@ -2181,7 +2181,8 @@ them, but it has a few caveats:
   ``batch_size``.
 * Updating fields defined on multi-table inheritance ancestors will incur an
   extra query per ancestor.
-* If ``objs`` contains duplicates, only the first one is updated.
+* When an individual batch contains duplicates, only the first instance in that
+  batch will result in an update.
 
 The ``batch_size`` parameter controls how many objects are saved in a single
 query. The default is to update all objects in one batch, except for SQLite