add user docs for 'ta undo'

docs/src/SUMMARY.md

@@ -10,6 +10,7 @@
     * [Dates and Durations](./time.md)
     * [Configuration](./config-file.md)
     * [Environment](./environment.md)
+    * [Undo](./undo.md)
     * [Synchronization](./task-sync.md)
   * [Running the Sync Server](./running-sync-server.md)
 - [Internal Details](./internals.md)

docs/src/undo.md

@@ -0,0 +1,7 @@
+# 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.