add user docs for 'ta undo'

docs/src/undo.md

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+# Undo
+
+It's easy to make a mistake: mark the wrong task as done, or hit enter before noticing a typo in a tag name.
+The `ta undo` command makes it just as easy to fix the mistake, but effectively reversing the most recent change.
+Multiple invocations of `ta undo` can be used to undo multiple changes.
+
+The limit of this functionality is that changes which have been synchronized to the server (via `ta sync`) cannot be undone.