Add more info about token authorization requirements (#13732)

dev-tools/cherrypick_pr

@@ -28,7 +28,9 @@ This script does the following:
   remote
 * if the --create_pr flag is used, it uses the GitHub API to create the PR
   for you. Note that this requires you to have a Github token with the
-  public_repo scope in the `~/.elastic/github.token` file
+  public_repo scope in the `~/.elastic/github.token` file. This token
+  should be also authorized to Elastic organization so as to work with single-sign-on.
+  (see https://help.github.com/en/articles/authorizing-a-personal-access-token-for-use-with-saml-single-sign-on)
 
 Note that you need to take the commit hashes from `git log` on the
 from_branch, copying the IDs from Github doesn't work in case we squashed the