Clarify how to work with threads.

anntzer · anntzer · commit 3e0a73c8291d · 2019-06-15T17:12:42.000+02:00
diff --git a/doc/faq/howto_faq.rst b/doc/faq/howto_faq.rst
@@ -544,8 +544,9 @@ that affect certain artists.  Hence, if you work with threads, it is your
 responsibility to set up the proper locks to serialize access to Matplotlib
 artists.
 
-Note that (for the case where you are working with an interactive backend) most
-GUI backends *require* being run from the main thread as well.
+You may be able to work on separate figures from separate threads.  However,
+you must in that case use a *non-interactive backend* (typically Agg), because
+most GUI backends *require* being run from the main thread as well.
 
 .. _howto-contribute:
 
