User guide: Explain when code owner submit records in the index get stale
User guide: Explain when code owner submit records in the index get stale

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: I1e04c46636eb997c52b7622855d33adac0995fce

GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners