Comment 2 for bug 1690756

This change affects keystone but I think it makes sense to only track the fix against the oslo.cache library. There isn't much keystone can do for this fix since the options are generated from oslo.cache and rendered in the sample configuration file.

Improvement to the configuration option help text should positively impact each project using oslo.cache and not just keystone.

Based on the existing help text, it appears that the thread count is the determining factor of what to use. Morgan has some context around oslo.cache and I'm following up with him to see if he had a specific thread count in mind when he wrote that comment [0].

Would it be fair to update the comment with something along the lines of:

- Use oslo_cache.memcache_pool for environments running more than X threads
- Use dogpile.cache.memcached for environments running less than X threads
- Use dogpile.cache.memory for environments isolated to a single process or thread (not recommended for production usage!)

[0] https://github.com/openstack/oslo.cache/blob/67c023c0a7fad02608a4db69bf2c12d00cc8f109/oslo_cache/_opts.py#L32-L45