docs: document trailing underscore behavior

src/argh/assembling.py

@@ -409,6 +409,13 @@ def set_default_command(
        If the parser was created with ``add_help=True`` (which is by default),
        option name ``-h`` is silently removed from any argument.
 
+    .. note::
+
+       Function argument names ending with an underscore will have a single
+       trailing underscore removed before being converted to CLI arguments.
+       This allows CLI arguments to have names that would otherwise clash with
+       a reserved word or shadow a builtin.
+
     """
     func_signature = inspect.signature(function)