Update guide with instructions to disable spinner (#1480)

* Update guide with instructions to disable spinner

This is in the command-line help, but can't be found anywhere obvious in the documentation.

* Oops, delete trailing space

Author: jacobtolar

docs/example/basic.rst

@@ -399,7 +399,8 @@ Parallel mode
   - ``auto`` to limit it to CPU count,
   - or pass an integer to set that limit.
 - Parallel mode displays a progress spinner while running tox environments in parallel, and reports outcome of
-  these as soon as completed with a human readable duration timing attached.
+  these as soon as completed with a human readable duration timing attached. This spinner can be disabled by
+  setting the environment variable ``TOX_PARALLEL_NO_SPINNER`` to the value ``1``.
 - Parallel mode by default shows output only of failed environments and ones marked as :conf:`parallel_show_output`
   ``=True``.
 - There's now a concept of dependency between environments (specified via :conf:`depends`), tox will re-order the