Clarify fuzzer docs for GSC uploads

Bug: None
Change-Id: If0a5254df30d9d8df5010439789bc83cc988531d
Reviewed-on: https://chromium-review.googlesource.com/1076496
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562460}

testing/libfuzzer/efficient_fuzzer.md

@@ -179,6 +179,9 @@ Alternative and faster way is to use [gsutil] command line tool:
 gsutil -m rsync <path_to_corpus> gs://clusterfuzz-corpus/libfuzzer/<my_fuzzer_name>_static
 ```
 
+Note that if you upload the corpus to GCS, the `seed_corpus` attribute is not
+needed in your `fuzzer_test` definition.
+
 ### Corpus Minimization
 
 It's important to minimize seed corpus to a *small set of interesting inputs*