Add additional form validation examples

Fixes #24811.

This adds .valid-feedback to our custom styles and server side examples; previously we ommitted this to suggest you don't always need valid feedback. In addition, this adds examples of the .{valid|invalid}-tooltip classes with a new subsection in the Validation docs.

docs/4.0/components/forms.md

@@ -724,15 +724,21 @@ For custom Bootstrap form validation messages, you'll need to add the `novalidat
 When attempting to submit, you'll see the `:invalid` and `:valid` styles applied to your form controls.
 
 {% example html %}
-<form id="needs-validation" novalidate>
+<form class="needs-validation" novalidate>
   <div class="form-row">
     <div class="col-md-4 mb-3">
       <label for="validationCustom01">First name</label>
       <input type="text" class="form-control" id="validationCustom01" placeholder="First name" value="Mark" required>
+      <div class="valid-feedback">
+        Looks good!
+      </div>
     </div>
     <div class="col-md-4 mb-3">
       <label for="validationCustom02">Last name</label>
       <input type="text" class="form-control" id="validationCustom02" placeholder="Last name" value="Otto" required>
+      <div class="valid-feedback">
+        Looks good!
+      </div>
     </div>
     <div class="col-md-4 mb-3">
       <label for="validationCustomUsername">Username</label>
@@ -777,16 +783,19 @@ When attempting to submit, you'll see the `:invalid` and `:valid` styles applied
 // Example starter JavaScript for disabling form submissions if there are invalid fields
 (function() {
   'use strict';
-
   window.addEventListener('load', function() {
-    var form = document.getElementById('needs-validation');
-    form.addEventListener('submit', function(event) {
-      if (form.checkValidity() === false) {
-        event.preventDefault();
-        event.stopPropagation();
-      }
-      form.classList.add('was-validated');
-    }, false);
+    // Fetch all the forms we want to apply custom Bootstrap validation styles to
+    var forms = document.getElementsByClassName('needs-validation');
+    // Loop over them and prevent submission
+    var validation = Array.prototype.filter.call(forms, function(form) {
+      form.addEventListener('submit', function(event) {
+        if (form.checkValidity() === false) {
+          event.preventDefault();
+          event.stopPropagation();
+        }
+        form.classList.add('was-validated');
+      }, false);
+    });
   }, false);
 })();
 </script>
@@ -847,10 +856,16 @@ We recommend using client side validation, but in case you require server side,
     <div class="col-md-4 mb-3">
       <label for="validationServer01">First name</label>
       <input type="text" class="form-control is-valid" id="validationServer01" placeholder="First name" value="Mark" required>
+      <div class="valid-feedback">
+        Looks good!
+      </div>
     </div>
     <div class="col-md-4 mb-3">
       <label for="validationServer02">Last name</label>
       <input type="text" class="form-control is-valid" id="validationServer02" placeholder="Last name" value="Otto" required>
+      <div class="valid-feedback">
+        Looks good!
+      </div>
     </div>
     <div class="col-md-4 mb-3">
       <label for="validationServerUsername">Username</label>
@@ -933,6 +948,67 @@ Our example forms show native textual `<input>`s above, but form validation styl
 </form>
 {% endexample %}
 
+### Tooltips
+
+If your form layout allows it, you can swap the `.{valid|invalid}-feedback` classes for `.{valid|invalid}-tooltip` classes to display validation feedback in a styled tooltip. While
+
+{% example html %}
+<form class="needs-validation" novalidate>
+  <div class="form-row">
+    <div class="col-md-4 mb-3">
+      <label for="validationCustom01">First name</label>
+      <input type="text" class="form-control" id="validationCustom01" placeholder="First name" value="Mark" required>
+      <div class="valid-tooltip">
+        Looks good!
+      </div>
+    </div>
+    <div class="col-md-4 mb-3">
+      <label for="validationCustom02">Last name</label>
+      <input type="text" class="form-control" id="validationCustom02" placeholder="Last name" value="Otto" required>
+      <div class="valid-tooltip">
+        Looks good!
+      </div>
+    </div>
+    <div class="col-md-4 mb-3">
+      <label for="validationCustomUsername">Username</label>
+      <div class="input-group">
+        <div class="input-group-prepend">
+          <span class="input-group-text" id="inputGroupPrepend">@</span>
+        </div>
+        <input type="text" class="form-control" id="validationCustomUsername" placeholder="Username" aria-describedby="inputGroupPrepend" required>
+        <div class="invalid-tooltip">
+          Please choose a unique and valid username.
+        </div>
+      </div>
+    </div>
+  </div>
+  <div class="form-row">
+    <div class="col-md-6 mb-3">
+      <label for="validationCustom03">City</label>
+      <input type="text" class="form-control" id="validationCustom03" placeholder="City" required>
+      <div class="invalid-tooltip">
+        Please provide a valid city.
+      </div>
+    </div>
+    <div class="col-md-3 mb-3">
+      <label for="validationCustom04">State</label>
+      <input type="text" class="form-control" id="validationCustom04" placeholder="State" required>
+      <div class="invalid-tooltip">
+        Please provide a valid state.
+      </div>
+    </div>
+    <div class="col-md-3 mb-3">
+      <label for="validationCustom05">Zip</label>
+      <input type="text" class="form-control" id="validationCustom05" placeholder="Zip" required>
+      <div class="invalid-tooltip">
+        Please provide a valid zip.
+      </div>
+    </div>
+  </div>
+  <button class="btn btn-primary" type="submit">Submit form</button>
+</form>
+{% endexample %}
+
 ## Custom forms
 
 For even more customization and cross browser consistency, use our completely custom form elements to replace the browser defaults. They're built on top of semantic and accessible markup, so they're solid replacements for any default form control.