DocBook: add a comment about the need for DocBook 5.2.

diff --git a/src/Buffer.cpp b/src/Buffer.cpp
index b1d14d245c5b520d75d199b5672e653ca346875f..07287221685cbc37769c1a4f4e0699608160c9e2 100644 (file)
--- a/src/Buffer.cpp
+++ b/src/Buffer.cpp
@@ -2179,6 +2179,7 @@ Buffer::ExportStatus Buffer::writeDocBookSource(odocstream & os,
                                                    + mathmlNamespace
                                                    + " xmlns:xi=\"http://www.w3.org/2001/XInclude\""
                                                    + " version=\"5.2\"";
+               // Version 5.2 is required for formalgroup.
 
                os << "<" << from_ascii(tclass.docbookroot()) << " " << from_ascii(attributes) << ">\n";
        }