Docs: Add a little extra info on snapshot deletion [skip ci][ci skip] (…

…#2661)

docs/users/cli-usage.md

@@ -667,6 +667,8 @@ Restored database snapshot: /Users/rfay/workspace/d8git/.ddev/db_snapshots/d8git
 
 Snapshots are stored in the project's .ddev/db_snapshots directory, and the directory can be renamed as necessary. For example, if you rename the above d8git_20180801132403 directory to "working_before_migration", then you can use `ddev restore-snapshot working_before_migration`.
 
+To delete a snapshot, delete its folder from the .ddev/db_snapshots directory.  Snapshots are not removed from the filesystem by `ddev delete`. It is safe to remove all the snapshots with `rm -r .ddev/db_snapshots` if you no longer need the snapshots.
+
 There are some interesting consequences of restoring huge multi-gigabyte databases. Ddev may show the project as ready and started when in reality tables are still being loaded. You can see this behavior with `ddev logs -s db -f`.
 
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