faq: add entry about accidentally amended working copy

docs/FAQ.md

@@ -94,6 +94,17 @@ You'll then need to use `jj new --before` to create new commits
 and `jj move --to`
 to move new changes into the correct commits.
 
+### I accidentally amended the working copy. How do I move the new changes into its own commit?
+
+Use `jj obslog -p` to see how your working-copy commit has evolved. Find the
+commit you want to restore the contents to. Let's say the current commit (with
+the changes intended for a new commit) are in commit X and the state you wanted
+is in commit Y. Note the commit id (normally in blue at the end of the line in
+the log output) of each of them. Now use `jj new` to create a new working-copy
+commit, then run `jj restore --from Y --to @-` to restore the parent commit
+to the old state, and `jj restore --from X` to restore the new working-copy
+commit to the new state.
+
 [config]: config.md
 
 [gitignore]: https://git-scm.com/docs/gitignore