[gtk-doc] mkhtml2: add chunk parameters for <preface> chunks

[Stefan Sauer <stefkost src gnome org>]

commit 6b56aff9ff5fd0cea57db91f6ac0f1d113e07d3d
Author: Stefan Sauer <ensonic users sf net>
Date:   Sun Mar 25 21:08:35 2018 +0200

    mkhtml2: add chunk parameters for <preface> chunks
    
    Those were determined by dropping the 'id' and cehcking what docbook did.

 gtkdoc/mkhtml2.py |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gtkdoc/mkhtml2.py b/gtkdoc/mkhtml2.py
index 309cfff..b4d20d6 100644
--- a/gtkdoc/mkhtml2.py
+++ b/gtkdoc/mkhtml2.py
@@ -109,12 +109,16 @@ class ChunkParams(object):
 # TODO: look up the abbrevs and hierarchy for other tags
 # http://www.sagehill.net/docbookxsl/Chunking.html#GeneratedFilenames
 # https://github.com/oreillymedia/HTMLBook/blob/master/htmlbook-xsl/chunk.xsl#L33
+#
+# If not defined, we can just create an example without an 'id' attr and see
+# docbook xsl does.
 CHUNK_PARAMS = {
     'appendix': ChunkParams('app', 'book'),
     'book': ChunkParams('bk'),
     'chapter': ChunkParams('ch', 'book'),
     'index': ChunkParams('ix', 'book'),
     'part': ChunkParams('pt', 'book'),
+    'preface': ChunkParams('pr', 'book'),
     'sect1': ChunkParams('s', 'chapter'),
     'section': ChunkParams('s', 'chapter'),
 }