[ISC-support #19862] Update docs to reflect changes to acceptable cookie-algorithm values

This documentation inconsistency was brought up by a customer here: https://support.isc.org/Ticket/Display.html?id=19862

I believe that the new description for cookie-algorithm should reflect the current behavior. I'm not sure if it should mention that support for sha1 and sha256 has been removed. The following two commits are the relevant code changes to this section of docs: afa81ee4 a912f313