Make --ascii work with DocBook output too.

MANUAL.txt

@@ -646,8 +646,8 @@ Options affecting specific writers
 
 `--ascii`
 
-:   Use only ASCII characters in output.  Currently supported only
-    for HTML output (which uses numerical entities instead of
+:   Use only ASCII characters in output.  Currently supported only for
+    HTML and DocBook output (which uses numerical entities instead of
     UTF-8 when this option is selected).
 
 `--reference-links`

src/Text/Pandoc/App.hs

@@ -411,7 +411,10 @@ convertWithOpts opts = do
                     selfcontain = if optSelfContained opts && htmlFormat
                                   then makeSelfContained writerOptions
                                   else return
-                    handleEntities = if htmlFormat && optAscii opts
+                    handleEntities = if (htmlFormat ||
+                                         format == "docbook4" ||
+                                         format == "docbook5" ||
+                                         format == "docbook") && optAscii opts
                                      then toEntities
                                      else id
                 output <- f writerOptions doc'