Documented the migration guide to multikey

[iulianpascalau]

Description of the pull request (what is new / what has changed)

• documented the migration guide to multikey

Did you test the changes locally ?

• [X] yes
• [ ] no

Which category (categories) does this pull request belong to?

• [ ] document new feature
• [ ] update documentation that is not relevant anymore
• [X] add examples or more information about a component
• [ ] fix grammar issues
• [ ] other