Document that wxDataViewCtrl now always expands its last column

This has been changed back in 4156e1a5c9
and is a (mildly) incompatible change, so document it in the appropriate
change log section.

See #18295.

docs/changes.txt

@@ -94,6 +94,9 @@ Changes in behaviour which may result in build errors
   not long. Its return value hasn't changed, however, and is still always
   either true or false, so normally the existing code should continue to work.
 
+- Generic wxDataViewCtrl now always resizes its last column to fill all the
+  available space, as the GTK+ version always did.
+
 - configure only accepts the options it knows about now and doesn't silently
   ignore all the rest. If you get errors about unknown options, you may either
   specify --disable-option-checking argument to continue accepting them (which