better document some of the incompatibilities introduced by the Unicode changes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@48606 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -14,12 +14,18 @@ in the manual for more details but here are the most important incompatible
 changes:
 
 - Many wxWidgets functions taking "const wxChar *" have been changed to take
-  either "const wxString&" if they should accept both Unicode or ANSI strings
+  either "const wxString&" so that they accept both Unicode and ANSI strings.
   and the argument can't be NULL or "const char *" if the strings are always
-  ANSI but may be NULL.
+  ANSI but may be NULL. This change is normally backwards compatible except:
+
+  a) Virtual functions: derived classes versions must be modified to take
+     "const wxString&" as well to make sure that they continue to override the
+     base class version.
+
+  b) Passing NULL as argument: as NULL can't be unambiguously converted to
+     wxString, in many cases code using it won't compile any more and NULL
+     should be replaced with an empty string.
 
-- Some structure fields have been changed from "wxChar *" to "char *" too:
-  e.g. wxCmdLineEntryDesc fields.
 
 
 Changes in behaviour not resulting in compilation errors, please read this!