Various improvements to undo file code to make it more robust.
diff --git a/runtime/doc/undo.txt b/runtime/doc/undo.txt
index c0b9d12..a7ae173 100644
--- a/runtime/doc/undo.txt
+++ b/runtime/doc/undo.txt
@@ -267,10 +267,7 @@
The file text differs from when the undo file was written. This means
the undo file cannot be used, it would corrupt the text. This also
happens when 'encoding' differs from when the undo file was written.
-*E825* *E826* *E831*
- The undo file does not contain valid contents and cannot be used.
-*E827* The magic number at the end of the file was not found. This usually
- means the file was truncated.
+*E825* The undo file does not contain valid contents and cannot be used.
Writing an undo file may fail for these reasons:
*E828* The file to be written cannot be created. Perhaps you do not have