changes.src: document FLAT obj changes.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

Author: H. Peter Anvin

doc/changes.src

@@ -77,6 +77,9 @@ It is the production version of NASM since 2025.
 \b A new command line option \c{-LF} allows overriding \c{[LIST -]}
    directives.
 
+\b In the \c{obj} output format, allow a segment in the \c{FLAT}
+   pseudo-group to also belong to another (real) group. Used on OS/2.
+
 \H{cl-2.xx} NASM 2 Series
 
 The NASM 2 series added support for x86-64, and was the production