Add some shorthands for special keyFields overrides

[epmatsw]

Per the docs right now:

    AllProducts: {
      // Singleton types that have no identifying field can use an empty
      // array for their keyFields.
      keyFields: [],
    },
    Store: {
      // If you need to disable normalization, set the keyFields to false
      // and the object will be embedded in the parent
      keyFields: false,
    },

these two cases make sense mechanically, but they're kind of hard to remember. I end up looking at that page somewhat frequently just to make sure I'm picking the right behavior.

I think it'd be nice if these two cases could be something like keyFields: "singleton" and keyFields: "no-normalize" or something like that so that it's easier to remember what they're doing.

[phryneas]

Hmm, I'll have to think about this, but my immediate reaction is that we probably won't do people a favor with something like that:

The "singleton" case should be extremely rare in most schemas, to the point where it's probably almost always an antipattern or accidental misuse. So giving that pattern more visibility through it's own shorthand might be a bad idea.

Maybe we just need a different angle to explain this in the docs?

Something like

`keyFields` are used to create the cache key to uniquely identify your item in the cache.

Some examples:

* If you specify `keyFields: ["id"]` on a `Posts` type, your cache key will look like `"Posts:1"` for the post with `id` of `1`.
* If you specify `keyFields: ["id", "locale"]` on a `Posts` type, your cache key will look like `"Posts:1:en"` for the post with `id` of `1` and `locale` of `en`.
* If you specify `keyFields: ["title", "author", ["name"]]`, on a `Posts` type, your cache key will look like `"Posts:lorem ipsum:Bob"` for a post with the `title` of `"lorem ipsum"` written by an author named `"Bob"`
* If you specify `keyFields: []` on a `Posts` type, all your posts will have the same cache key of `"Posts"`. Warning: that means that if you have more than one post that you receive from your GraphQL server, those posts will overwrite each other and might be merged into each other.
  This might be useful for the very rare case where you have only one item of a specific type (a singleton), but most of the time you should try to avoid this as it might cause unintended data loss.
* If you don't want your cache items to have their own cache entry, but want them stored together with the next normalized parent, you can give them `keyFields: false`.

(Note that I oversimplified the cache keys in this example, a real docs entry would have the correct format ^^)

What do you think? Would that make it easier to remember?