Rewrite input group component (#25020)

* Rewrite input group component

* Set the feedback to 100% width for input group

* Move from .row to .form-row for tighter layout

* no need for custom feedback here, we're using browser messaging

* add input group to validation examples

* add note about validating multiple

* migration note added

docs/4.0/migration.md

@@ -16,6 +16,16 @@ While Beta 2 saw the bulk of our breaking changes during the beta phase, but we
 - Updated selector for input-based button groups. Instead of `[data-toggle="buttons"] { }` for style and behavior, we use the `data` attribute just for JS behaviors and rely on a new `.btn-group-toggle` class for styling.
 - 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.
 
+### 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.
+
+- Validation styles are now supported, as are multiple inputs (though you can only validate one input per group).
+
+- Sizing classes must be on the parent `.input-group` and not the individual form elements.
+
+- Due to limitations in how CSS selectors work, all buttons must be the same element (e.g., `<a>` or `<button>`).
+
 ## Beta 2 changes
 
 While in beta, we aim to have no breaking changes. However, things don't always go as planned. Below are the breaking changes to bear in mind when moving from Beta 1 to Beta 2.