ST 2.1 docs: Add info for isUnique flag and Model uuid config setting

[jep]

This is just a reminder that the Model docs need to be updated to describe both the isUnique flag and the

identifier:{
    type:'uuid'
}

config setting. Neither appear in the docs in ST 2.1 RC2, but you are given warnings to use them.

Edit: Upon digging, I did find that "identifier" is in Model, and it does refer to a uuid strategy. But it doesn't explicitly say to use the string "uuid". Perhaps it would be better to explicitly give the "{type:'uuid'}" example.

Edit2: And upon even further digging, it seems like if the help for identifier would just link to Ext.data.identifier.Simple and tell you to use that or a subclass, it'd be easy to understand.

[mitchellsimoens]

Thanks for the report! I have opened a bug in our bug tracker.