flux-overlay(1): offline errors are now shown

Problem: flux-overlay(1) states that 'flux overlay errors' summarizes
errors for lost nodes, but now offline nodes can also be included.

Update text.

Author: garlick

doc/man1/flux-overlay.rst

@@ -86,8 +86,9 @@ errors
 
 .. program:: flux overlay errors
 
-:program:`flux overlay errors` summarizes any errors recorded for lost nodes.
-The output consists of one line per unique error with a hostlist prefix.
+:program:`flux overlay errors` summarizes any errors recorded for lost or
+offline nodes.  The output consists of one line per unique error with a
+hostlist prefix.
 
 .. option:: -t, --timeout=FSD