docs(rtk-query): Clarify standard slice exports (#4833)

The existing documentation implies that standard slices *only* export
action creators and reducers. This can create confusion when users
compare this with fuller implementations within the codebase, such as
the `postsSlice.ts` example shown earlier in Part 6.

That file demonstrates that robust slices require exporting additional
items like async thunks, types, selectors, and listener middleware.

This change updates the wording to clarify that the original statement
refers to the *minimal* exports needed from `createSlice`, providing
better and unambiguous context before contrasting with `createApi`
exports.