JS: Expand doc string in installFromTarballurl

Co-authored-by: Erik Krogh Kristensen <erik-krogh@github.com>

javascript/extractor/src/com/semmle/js/dependencies/AsyncFetcher.java

@@ -89,6 +89,8 @@ public class AsyncFetcher {
     /**
      * Extracts the relevant contents of the given tarball URL in the given folder;
      * the returned future completes when done.
+     *
+     * If the same tarball has already been extracted elsewhere, then symbolic links are added to `destDir` linking to the already extracted tarball. 
      */
     public CompletableFuture<Void> installFromTarballUrl(String tarballUrl, Path destDir) {
         return tarballExtractions.get(tarballUrl, () -> {