Document this feature.

From Glance OSP 18 features reviews: https://docs.google.com/document/d/1-kBXyAHisuzgRrzsF42k2A3W-4eqbayx5HvPTbEq66c/edit#heading=h.h9pbdrdbs2rs

Does the feature have documentation impact and require early planning with doc team?

Explain the feature and list the new commands. Mention that the admin must connect to the api node to manage the cache (if we don’t improve the glance cli). Document API workaround if needed.