Possible/probable to add conflicting rules with multi-select checkboxes element

## Description
Branching assumes that choices are exclusive. With checkboxes, multiple selections are possible and some will override others. Is there a way to help guide users through this.

## Steps to reproduce
1. Go to GC Forms
2. Add 4 new pages
3. On the first page add a checkbox question with three options
4. Have each option branch to a different page
5. Test form with multiple selections

## Details
- **Browser**: Chrome
- **Operating system**: Mac
- **Environment**: Staging
- **Language**: English
- **Viewport**: Desktop

## Expected behaviour
Should branching only be possible for single-select options?
Error messages to prevent common errors with multi-select?
Possibility to combine options in future (if select 1 AND 2, then show page Y)?

## Screenshots or videos