@xref{Query Before Exit}.
@item :stop @var{stopped}
-@var{stopped} must be @code{nil}. The @code{:stop} key is ignored
-otherwise and is retained for compatibility with other process types
-such as pipe processes. Asynchronous subprocesses never start in the
-stopped state.
+If provided, @var{stopped} must be @code{nil}; it is an error to use
+any non-@code{nil} value. The @code{:stop} key is ignored otherwise
+and is retained for compatibility with other process types such as
+pipe processes. Asynchronous subprocesses never start in the stopped
+state.
@item :filter @var{filter}
Initialize the process filter to @var{filter}. If not specified, a