docs: mention "jj file untrack" in Git compatibility list

Closes #5059

docs/git-compatibility.md

@@ -31,10 +31,13 @@ a comparison with Git, including how workflows are different, see the
   and [how they interoperate with Git](#branches).
 * **Tags: Partial.** You can check out tagged commits by name (pointed to be
   either annotated or lightweight tags), but you cannot create new tags.
-* **.gitignore: Yes.** Ignores in `.gitignore` files are supported. So are
+* **.gitignore: Yes.** Patterns in `.gitignore` files are supported. So are
   ignores in `.git/info/exclude` or configured via Git's `core.excludesfile`
-  config. The `.gitignore` support uses a native implementation, so please
-  report a bug if you notice any difference compared to `git`.
+  config. Since working-copy files are snapshotted by every `jj` command, you
+  might need to run `jj file untrack` to exclude newly ignored files from the
+  working-copy commit. It's recommended to set up the ignore patterns earlier.
+  The `.gitignore` support uses a native implementation, so please report a bug
+  if you notice any difference compared to `git`.
 * **.gitattributes: No.** There's [#53](https://github.com/martinvonz/jj/issues/53)
   about adding support for at least the `eol` attribute.
 * **Hooks: No.** There's [#405](https://github.com/martinvonz/jj/issues/405)