make note of extension.worktreeConfig

It came up in starship: starship/starship#3245

Author: Byron

Diff for: src/plumbing/progress.rs

@@ -332,6 +332,12 @@ static GIT_CONFIG: &[Record] = &[
             ),
         },
     },
+    Record {
+        config: "extensions.worktreeconfig",
+        usage: Planned {
+            note: Some("Seems to be turned on when sparse indices are used")
+        },
+    },
     Record {
         config: "committer.name",
         usage: InModule {