[ruby/irb] Add help about extra doc dir option

https://github.com/ruby/irb/commit/5018f2cb99

lib/irb/lc/help-message

@@ -20,6 +20,7 @@ Usage:  irb.rb [options] [programfile] [arguments]
   -W[level=2]       Same as `ruby -W`
   --context-mode n  Set n[0-4] to method to create Binding Object,
                     when new workspace was created
+  --extra-doc-dir   Add an extra doc dir for the doc dialog
   --echo            Show result (default)
   --noecho          Don't show result
   --echo-on-assignment

man/irb.1

@@ -105,6 +105,10 @@ Uses singleline editor module.
 Doesn't use singleline editor module.
 .Pp
 .Pp
+.It Fl -extra-doc-dir
+Add an extra doc dir for the doc dialog.
+.Pp
+.Pp
 .It Fl -echo
 Show result (default).
 .Pp