[DOC] upgrade section needs an explanation of changes to Bucket Cache

the first pass version of the upgrade docs has a brief note about the bucket cache changing:

• hbase.bucketcache.combinedcache.enabled <!-- This seems worth a call out section? -->
• hbase.bucketcache.ioengine no longer supports the 'heap' value. <!-- This seems worth a call out section? -->

But the changes are substantial and warrant a section of their own under the "Changes of Note!" banner.