Use full path in undofile(). Updated docs.
diff --git a/runtime/doc/undo.txt b/runtime/doc/undo.txt
index e8fed4f..6e2308f 100644
--- a/runtime/doc/undo.txt
+++ b/runtime/doc/undo.txt
@@ -314,6 +314,16 @@
If it is zero, the Vi-compatible way is always used. If it is negative no
undo is possible. Use this if you are running out of memory.
+ *clear-undo*
+When you set 'undolevels' to -1 the undo information is not immediately
+cleared, this happens at the next change. To force clearing the undo
+information you can use these commands: >
+ :let old_undolevels = &undolevels
+ :set undolevels=-1
+ :exe "normal a \<BS>\<Esc>"
+ :let &undolevels = old_undolevels
+ :unlet old_undolevels
+
Marks for the buffer ('a to 'z) are also saved and restored, together with the
text. {Vi does this a little bit different}