Clarify help string for --group-by

augur/filter.py

@@ -1149,7 +1149,7 @@ def register_arguments(parser):
     sequence_filter_group.add_argument('--non-nucleotide', action='store_true', help="exclude sequences that contain illegal characters")
 
     subsample_group = parser.add_argument_group("subsampling", "options to subsample filtered data")
-    subsample_group.add_argument('--group-by', nargs='+', help="categories with respect to subsample; two virtual fields, \"month\" and \"year\", are supported if they don't already exist as real fields but a \"date\" field does exist")
+    subsample_group.add_argument('--group-by', nargs='+', help="categories with respect to subsample; If a \"date\" field exists, two generated fields \"month\" and \"year\" are also available and will override any fields with the same name.")
     subsample_limits_group = subsample_group.add_mutually_exclusive_group()
     subsample_limits_group.add_argument('--sequences-per-group', type=int, help="subsample to no more than this number of sequences per category")
     subsample_limits_group.add_argument('--subsample-max-sequences', type=int, help="subsample to no more than this number of sequences; can be used without the group_by argument")