docs: add pruning-related warning messages in MongoDB connection

We migrated the source of truth for what the active draft and published
versions of course and v1 library content are to the
SplitModulestoreCourseIndex Django model. But the contentpruning
code (structures.py) that was developed in tubular and will be moved to
edx-platform is not aware of this newer model, and still only pulls its
source of truth from MongoDB. So we *must* continue to do writes to
MongoDB, or the pruning code will start pruning live versions.

The longer term fix for this is to make the pruning code aware of
SplitModulestoreCourseIndex, which will be easier once it's moved into
edx-platform.

xmodule/modulestore/split_mongo/mongo_connection.py

@@ -701,7 +701,9 @@ class DjangoFlexPersistenceBackend(MongoPersistenceBackend):
         course_index['last_update'] = datetime.datetime.now(pytz.utc)
         new_index = SplitModulestoreCourseIndex(**SplitModulestoreCourseIndex.fields_from_v1_schema(course_index))
         new_index.save()
-        # TEMP: Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well:
+        # Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well.
+        # NOTE: This is REQUIRED for pruning (structures.py) to run safely. Don't remove this write until
+        # pruning is modified to read from SplitModulestoreCourseIndex to get active versions.
         super().insert_course_index(course_index, course_context)
 
     def update_course_index(self, course_index, from_index=None, course_context=None):  # pylint: disable=arguments-differ
@@ -755,7 +757,10 @@ class DjangoFlexPersistenceBackend(MongoPersistenceBackend):
 
         # Save the course index entry and create a historical record:
         index_obj.save()
-        # TEMP: Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well:
+
+        # Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well.
+        # NOTE: This is REQUIRED for pruning (structures.py) to run safely. Don't remove this write until
+        # pruning is modified to read from SplitModulestoreCourseIndex to get active versions.
         super().update_course_index(course_index, from_index, course_context)
 
     def delete_course_index(self, course_key):
@@ -764,7 +769,9 @@ class DjangoFlexPersistenceBackend(MongoPersistenceBackend):
         """
         RequestCache(namespace="course_index_cache").clear()
         SplitModulestoreCourseIndex.objects.filter(course_id=course_key).delete()
-        # TEMP: Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well:
+        # Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well.
+        # NOTE: This is REQUIRED for pruning (structures.py) to run safely. Don't remove this write until
+        # pruning is modified to read from SplitModulestoreCourseIndex to get active versions.
         super().delete_course_index(course_key)
 
     def _drop_database(self, database=True, collections=True, connections=True):
@@ -782,5 +789,7 @@ class DjangoFlexPersistenceBackend(MongoPersistenceBackend):
                 "post-test cleanup failed with TransactionManagementError. "
                 "Use 'with self.allow_transaction_exception():' from ModuleStoreTestCase/...IsolationMixin to fix it."
             ) from err
-        # TEMP: Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well:
+        # Also write to MongoDB, so we can switch back to using it if this new MySQL version doesn't work well.
+        # NOTE: This is REQUIRED for pruning (structures.py) to run safely. Don't remove this write until
+        # pruning is modified to read from SplitModulestoreCourseIndex to get active versions.
         super()._drop_database(database, collections, connections)