Documentation: set a !DOCTYPE for user manual

asciidoc already takes care of including a doctype for most of the
HTML documentation, but the user manual which is processed with
docbook-xsl directly lacks one (at least with Debian docbook-xsl
1.75.2+dfsg-5).  This makes it harder to automatically validate the
HTML.

Reported-by: 積丹尼 <jidanni@jidanni.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/docbook.xsl

@@ -1,5 +1,8 @@
 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 		version='1.0'>
  <xsl:import href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"/>
- <xsl:output method="html" encoding="UTF-8" indent="no" />
+ <xsl:output method="html"
+     encoding="UTF-8" indent="no"
+     doctype-public="-//W3C//DTD HTML 4.01//EN"
+     doctype-system="http://www.w3.org/TR/html4/strict.dtd" />
 </xsl:stylesheet>