What is the Vuex “context” object?

From the documentation you pointed out you can read:

We will see why this context object is not the store instance itself when we introduce Modules later.

The main idea of the context object is to abstract the scope of the current Module. If you simply access store.state, it will always be the root state.

The context object of actions and its properties/methods are described here in the source code and also referenced in the API documentation

Here is the list:

{
  state,      // same as store.state, or local state if in modules
  rootState,  // same as store.state, only in modules
  commit,     // same as store.commit
  dispatch,   // same as store.dispatch
  getters,    // same as store.getters, or local getters if in modules
  rootGetters // same as store.getters, only in modules
}