[LyX/master] DocBook: add a comment about the need for DocBook 5.2.

[Thibaut Cuvelier]

commit ff2a2b2a8a3be323346ad4f5e660fbc1b7a65a04
Author: Thibaut Cuvelier <tcuvelier at lyx.org>
Date:   Fri Dec 23 02:22:55 2022 +0100

    DocBook: add a comment about the need for DocBook 5.2.
---
 src/Buffer.cpp |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/src/Buffer.cpp b/src/Buffer.cpp
index b1d14d2..0728722 100644
--- 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";
 	}