Update comment in additionalAuthenticationChecks to clarify why our no-op implementation is ok

[dwnusbaum]

While investigating a security issue a few months back, I noticed that our implementation of AbstractUserDetailsAuthenticationProvider.additionalAuthenticationChecks did not use the approach recommended in the superclass method's Javadoc, which was a bit concerning after looking at some of the branches in this code. After some investigation it seems fine, but I think it is worth noting why it is fine in case someone copies this code when creating a new security realm and they want to use Spring Security's user caching system.

Testing done

This PR only updates a comment.

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[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

/label ready-for-merge