Document the change to breakpoint mixins (#34251)
Document the change to breakpoint mixins (#34251)

* Document the change to breakpoint mixins

* Update migration.md

Co-authored-by: XhmikosR <b60f67e10123951b393d9ba15dcf79efc14744bb@gmail.com>
m5o/bootstrap,inway/bootstrap,m5o/bootstrap,twbs/bootstrap,inway/bootstrap,tjkohli/bootstrap,coliff/bootstrap,twbs/bootstrap,inway/bootstrap,coliff/bootstrap,tjkohli/bootstrap