Added information about the help-block class

Fixes #18505
Closes #18509

docs/migration.md

@@ -76,6 +76,7 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 - Renamed `.control-label` to `.form-control-label`.
 - Renamed `.input-lg` and `.input-sm` to `.form-control-lg` and `.form-control-sm`, respectively.
 - Dropped `.form-group-*` classes for simplicity's sake. Use `.form-control-*` classes instead now.
+- Dropped `.help-block`. Use the `.text-muted` utility class instead.
 - Horizontal forms overhauled:
   - Dropped the `.form-horizontal` class requirement.
   - `.form-group` no longer mixins the `.row` class, so it's now required for grid layouts.