Migration docs: Mention $grid-breakpoints and $container-max-widths by name

[skip sauce]

docs/migration.md

@@ -34,7 +34,7 @@ Here are the big ticket items you'll want to be aware of when moving from v3 to
 - Overhauled grid mixins to merge `make-col-span` into `make-col` for a singular mixin.
 - Added a new `sm` grid tier below `768px` for more granular control. We now have `xs`, `sm`, `md`, `lg`, and `xl`. This also means every tier has been bumped up one level (so `.col-md-6` in v3 is now `.col-lg-6` in v4).
 - Changed grid system media query breakpoints and container widths to account for new grid tier and ensure columns are evenly divisible by `12` at their max width.
-- Grid breakpoints and container widths are now handled via Sass maps instead of a handful of separate variables. These replace the `@screen-*` variables entirely and allow you to fully customize the grid tiers.
+- Grid breakpoints and container widths are now handled via Sass maps (`$grid-breakpoints` and `$container-max-widths`) instead of a handful of separate variables. These replace the `@screen-*` variables entirely and allow you to fully customize the grid tiers.
 - Media queries have also changed. Instead of repeating our media query declarations with the same value each time, we now have `@include media-breakpoint-up/down/only`. Now, instead of writing `@media (min-width: @screen-sm-min) { ... }`, you can write `@include media-breakpoint-up(sm) { ... }`.
 
 ### Components