Add example of new feature for mutually exclusive elements

[costateixeira]

if you write a constraint on an element that follows this pattern:

(name | name | name).count = 1

(or <= 1)
where all the names in the union are children of the element on which this invariant sits, then the IG publisher will understand that the named elements are a mutually exclusive group. That means that:

• They will be rendered as a mutually exclusive group in the table presentation form
• if you make one required in a profile, the others will automatically be set to max = 0 (that must be true, so it saves you from having to be explicit about it)
• the rendering as a group will collapse out if one of the elements is required

I added this to reduce the comprehension impedence for CCDA guide readers