.text-help is dead, long live .form-control-feedback

Uses a new more specific class, adds some basic spacing, and clarifies
that this class is only for validation feedback.

Clears up the help text class question in
https://github.com/twbs/bootstrap/pull/18704#issuecomment-168241731 and
addresses the vertical spacing in #18703.

scss/_forms.scss

@@ -265,6 +265,10 @@ input[type="checkbox"] {
 //
 // Apply contextual and semantic states to individual form controls.
 
+.form-control-feedback {
+  margin-top: .5rem;
+}
+
 .form-control-success,
 .form-control-warning,
 .form-control-danger {

scss/mixins/_forms.scss

@@ -5,7 +5,7 @@
 
 @mixin form-control-validation($color) {
   // Color the label and help text
-  .text-help,
+  .form-control-feedback,
   .form-control-label,
   .radio,
   .checkbox,