Assume keyArgs:false only if *both* read and merge provided.

[benjamn]

Follow-up to #5680.

Assuming keyArgs:false just because a merge function (or a read function) was defined means that the following pattern can actually change cache behavior in surprising ways:

const cache = new InMemoryCache({
  typePolicies: {
    Person: {
      fields: {
        friends: {
          // This is supposed to be identical to what the cache does by
          // default, except without warnings about data loss, but it
          // subtly alters the default argument-based field identity
          // behavior when arguments are provided to the friends field.
          merge(_, incoming) {
            return incoming;
          },
        },
      },
    },
  },
});

Although this merge function mirrors how the cache behaves without a merge function, it has the benefit of making that behavior explicit, thereby silencing the warnings introduced by #5833. In order to recommend this strategy for silencing warnings when a last-write-wins merge strategy is desired, it seems unacceptable for this pattern to have any unexpected consequences beyond silencing the warnings.

Thinking about this default keyArgs:false behavior more, disabling field identity with keyArgs:false doesn't really make sense if you have only a merge function, or only a read function, since any fancy argument handling done by one of those functions cannot be balanced in the other direction.

[stackia]

I hope this behavior should be clarified at https://www.apollographql.com/docs/react/caching/cache-field-behavior/#specifying-key-arguments