Fix docstring

Author: benjeffery

python/tskit/trees.py

@@ -10556,9 +10556,11 @@ def map_to_vcf_model(
         and the specified ploidy.
 
         If neither `name_metadata_key` nor `individual_names` is not specified, the
-        individual names are set to "tsk_{individual_id}" for each individual.
+        individual names are set to "tsk_{individual_id}" for each individual. If
+        no individuals are present, the individual names are set to "tsk_{i}" with
+        `0 <= i < num_sample_nodes/ploidy`.
 
-        Warnings are emmitted if any sample nodes do not have an individual ID.
+        A Warning are emmitted if any sample nodes do not have an individual ID.
 
         :param list individuals: Specific individual IDs to include in the VCF. If not
             specified and the tree sequence contains individuals, all individuals are