document replace_one's reply

Author: kmahar

src/libmongoc/doc/mongoc_collection_replace_one.rst

@@ -34,7 +34,7 @@ Description
 
 This function shall replace documents in ``collection`` that match ``selector`` with ``replacement``.
 
-If you pass a non-NULL ``reply``, it is filled out with fields "modifiedCount" and "matchedCount". If there is a server error then ``reply`` contains either a "writeErrors" array with one subdocument or a "writeConcernErrors" array. The reply must be freed with :symbol:`bson:bson_destroy`.
+If provided, ``reply`` will be initialized and populated with the fields ``matchedCount``, ``modifiedCount``, ``upsertedCount``, and optionally ``upsertedId`` if applicable. If there is a server error then ``reply`` contains either a ``writeErrors`` array with one subdocument or a ``writeConcernErrors`` array. The reply must be freed with :symbol:`bson:bson_destroy`.
 
 See Also
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