fixes #4909: add color utility classes .text-warning, etc and document them

less/type.less

@@ -32,9 +32,23 @@ em {
 cite {
   font-style: normal;
 }
+
+// Utility classes
 .muted {
   color: @grayLight;
 }
+.text-warning {
+  color: @warningText;
+}
+.text-error {
+  color: @errorText;
+}
+.text-info {
+  color: @infoText;
+}
+.text-success {
+  color: @successText;
+}
 
 
 // Headings