Add list of options for aitf-update-cache --products

https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/435

example/aitf/argparsetools.py

@@ -90,7 +90,7 @@ def add_products_arg(parser):
     parser.add_argument('--products', dest='product_string', 
         default='all',
         metavar='PRODUCT_LIST',
-        help='comma separated list of products to retrieve inputs for (default: %(default)s))')
+        help=f'comma separated list of products to retrieve inputs for; options include {", ".join(KNOWN_PRODUCTS)} (default: %(default)s))')
 
 def process_products(args):
     import logging