Add a short info card to the target modifiers screen to explain API decisions

[damonhook]

Following from a discussion in #60

Now that only one save-after-save mod can be rolled, it is not clear that you should add target modifiers as they appear on the warscroll (including duplicate FNP / MW Negate modifiers).

The same thing is true for re-rolls. If you add a "re-roll ones", and a "re-roll all", the API will use the "re-roll all" as it is the best choice (as re-rolls don't stack)

The backend API actually picks which is the best modifier to use for a certain scenario (so the user doesn't need to worry about it)

We should add a short info card to the screen to explain this. Something along the lines of:

Modifiers should be added as they appear on the warscroll. The application will apply the best modifiers for the scenario (obeying the rules for Age of Sigmar).