Document use of additional modal classes in `size` property.

[alanmies]

There are some Bootstrap 4 classes one might want to include in dialogs, such as "modal-dialog-centered" and "modal-dialog-scrollable" - short of rewriting the template code, those can be easily injected by including them in the "size" parameter for $uibModal.open. Like "size: 'md modal-dialog-centered'". So include this in the documentation as well?

[Morgul]

@alanmies That's a great idea. While adding two more properties for this wouldn't be all that hard, this change makes for a pretty nice work around until/if that happens.