docs: add default to repository_cache flag help

The docs currently state "default: see description" for the `--repository_cache` flag, but the description doesn't state the default, just the use of an empty value.

Closes bazelbuild#17225.

PiperOrigin-RevId: 511162777
Change-Id: Ib0a4a1331a0e54e37726a54962a665df092fd38b

src/main/java/com/google/devtools/build/lib/bazel/repository/RepositoryOptions.java

@@ -43,7 +43,8 @@ public class RepositoryOptions extends OptionsBase {
       help =
           "Specifies the cache location of the downloaded values obtained "
               + "during the fetching of external repositories. An empty string "
-              + "as argument requests the cache to be disabled.")
+              + "as argument requests the cache to be disabled, "
+              + "otherwise the default of '<output_user_root>/cache/repos/v1' is used")
   public PathFragment experimentalRepositoryCache;
 
   @Option(