update docstring.

Author: WaVEV

django_mongodb/compiler.py

@@ -399,7 +399,8 @@ def build_query(self, columns=None):
                 )
             query.combinator_pipeline = self.get_combinator_queries()
         else:
-            # If we query distinct
+            # If query is distinct, build a $group stage for distinct fields,
+            # then set project fields based on grouped _id
             if self.query.distinct:
                 distinct_fields = self.get_project_fields(
                     columns, ordering_fields, force_expression=True
@@ -409,6 +410,7 @@ def build_query(self, columns=None):
                 query.aggregation_pipeline.append({"$group": {"_id": distinct_fields}})
                 query.project_fields = {key: f"$_id.{key}" for key in distinct_fields}
             else:
+                # Otherwise, simply project fields without grouping
                 query.project_fields = self.get_project_fields(columns, ordering_fields)
             # If columns is None, then get_project_fields() won't add
             # ordering_fields to $project. Use $addFields (extra_fields) instead.