docs(withResource): mention how to patch resource state

[michael-small]

Wondered how, imagined this was how, and saw that the JSDoc mentions this. But no doc mention until now.

Closes #281

[rainerhahnekamp]

For both named and unnamed: Maybe we don't want to update a resource in a non-resolved status. So I wouldn't use undefined or `[];

What about something in the direction of
patchState(store, { value: { id: 1, name: 'John' } });

patchState(store, ({ listValue }) => { listValue: [...listValue, { id: 1, name: 'John' }]})

[rainerhahnekamp]

Will you apply the changes we discussed? Explaining the impact that undefined value vs. previous value has?

[rainerhahnekamp]

Thanks @michael-small