add more details about the podresources API guarantees

**This is a Feature Request**

**What would you like to be added**
Document the guarantees the Podresources API provides and what consumers can expect with more details

**Why is this needed**
Users of the API have a time harder than necessary figuring out what the API promises and the semantics of some fields as partially highlighed in https://github.com/kubernetes-sigs/node-feature-discovery/pull/2509 - most notably, some guarantees provided by the code and intentional in design didn't surface in the official docs, or aren't clear enough

**Comments**
N/A