Documentation: unbreak user-manual.

The previous one broke generated xml files for anything but manpages,
as it took the header for manpage unconditionally.  This fixes it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/asciidoc.conf

@@ -31,6 +31,7 @@ ifdef::backend-docbook[]
 {title#}</example>
 endif::backend-docbook[]
 
+ifdef::doctype-manpage[]
 ifdef::backend-docbook[]
 [header]
 template::[header-declarations]
@@ -47,6 +48,7 @@ template::[header-declarations]
   <refpurpose>{manpurpose}</refpurpose>
 </refnamediv>
 endif::backend-docbook[]
+endif::doctype-manpage[]
 
 ifdef::backend-xhtml11[]
 [gitlink-inlinemacro]