updates phx.gen.auth docs and mix task help info

[Morzaram]

Hey there,

This is my first time doing this thing and I saw some information was missing in regards to doing multiple accounts within phx.gen.auth. I believe this is the best way to do this hence my update of the documentation. However, I'm not the most advanced writer, nor developer. So if you have any issues then that's okay by me. I do think that some of the mix tasks should cover this though since contexts and scaffolders are powerful but not too much is there when it comes to scaffolding into folders as I cover in the paragraphs I've written.

Thank you and I hope you find this useful!