Clarify usage of getters and actions

[LeoColman]

Hi guys!

I'm a newcomer to Vue, Vuetify, Vuex and Cognito (backender playing a little bit), and I had A LOT of troubles to get this library working on my end.

This happened because I didn't know that I must first dispatch fetchSession before any variable is available. I finally found that I needed to do it from the example repository, and it was kind of hidden there too.

Now, I don't know if this confusion is because of my poor experience with all these tools, or if the documentation can be indeed improved.