Address feedback on pagination examples raised in #5881.

[benjamn]

Thanks to @tafelito for actually trying out the code examples in the new cache policies documentation (see #5881)! I have simplified the suggested edits to the examples, but (I hope) preserved their intent.

As for the TypeScript changes, we were previously defaulting TExisting to StoreValue, and TIncoming to TExisting, but that was never really helpful (and sometimes just frustrating), because the definition of the StoreValue type reads like an incomplete list of some of the types you might find inside the cache:

export type StoreValue =
  | number
  | string
  | string[]
  | Reference
  | Reference[]
  | null
  | undefined
  | void
  | Object;

When you're writing a specific read or merge function, you almost always have a much better idea what type of data you're dealing with, so there's pretty much zero value in thinking through these cases, when you can just name your own parameter types and be done with it.

This is one of those rare cases when defaulting to any is a good option, because it places the responsibility on the developer to handle all possible cases, using their application-level knowledge.

[hwillson]

👍 @benjamn (and thanks @tafelito!).