guide: update Ext Data guide link to add to-cache/remote examples

per #2246 (comment)

content/docs/user-guide/managing-external-data.md

@@ -2,9 +2,10 @@
 
 > ⚠️ This is an advanced feature that we don't recommend using unless you really
 > know what you are doing. Artifacts added with --external are not affected by
-> `dvc push/pull/status -c`. You are likely looking for
-> [straight-to-remote/cache](https://github.com/iterative/dvc/issues/4520)
-> functionality or `dvc import-url`
+> `dvc push/pull/status -c`. You are likely looking for straight
+> [to-cache](/doc/command-reference/add#example-transfer-to-the-cache) or
+> [to-remote](/doc/command-reference/add#example-transfer-to-remote-storage)
+> transfers, or `dvc import-url`).
 
 There are cases when data is so large, or its processing is organized in such a
 way, that its preferable to avoid moving it from its original location. For