tests/screentest/run.sh: explain more how to use ci

Change-Id: I61bc941d338a903021a838761a11d5cf8f8aefc1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/384634
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
diff --git a/tests/screentest/run.sh b/tests/screentest/run.sh
index c4aff77..7c91a03 100755
--- a/tests/screentest/run.sh
+++ b/tests/screentest/run.sh
@@ -17,6 +17,8 @@
   [ci]
     Run tests against a local server with a seeded database. This is what runs in
     CI/kokoro and should always pass on master.
+    Note: on first run, create a seeded database with --seeddb. It will persist
+    for subsequent runs unless --rm is also provided.
 
   [local]
     Run tests against a local server started with ./devtools/run_local.sh <env>
@@ -82,7 +84,7 @@
      "--idtoken")
         shift
         idtoken=$1
-        ;; 
+        ;;
       "--seeddb")
         seeddb=true
         ;;