Mention -file-text change in Migration docs (#25144)

Fixes #25127.

Author: mdo

docs/4.0/migration.md

@@ -27,6 +27,8 @@ While Beta 2 saw the bulk of our breaking changes during the beta phase, but we
 
 - Removed `.col-form-legend` in favor of a slightly improved `.col-form-label`. This way `.col-form-label-sm` and `.col-form-label-lg` can be used on `<legend>` elements with ease.
 
+- Custom file inputs received a change to their `$custom-file-text` Sass variable. It's no longer a nested Sass map and now only powers one string—the `Browse` button as that is now the only pseudo-element generated from our Sass. The `Choose file` text now comes from the `.custom-file-label`.
+
 ### Input groups
 
 - Input group addons are now specific to their placement relative to an input. We've dropped `.input-group-addon` and `.input-group-btn` for two new classes, `.input-group-prepend` and `.input-group-append`. You must explicitly use an append or a prepend now, simplifying much of our CSS.