[docs][NodeResourceTopology] make sure cache implementations are described in the KEP #561
Description
We need to make sure that discardreservednodes and overreserve caches are outlined in the main plugin KEP. It's totally fine to defer detailed design to specific docs and code comments, like we already have, but purpose, goals and high level design should be succintly described in the KEP, which is the entry point of the documentation for people which want to understand the features. Overreserve surely needs some work here.