minor #15237 Adding label_html for asterisk (ThomasLandauer)

This PR was submitted for the 5.2 branch but it was squashed and merged into the 4.4 branch instead.

Discussion
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Adding label_html for asterisk

Closes #14915

I hope I got the link right - please double-check :-)

Commits
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a7ad737 Adding label_html for asterisk

Author: javiereguiluz

form/bootstrap4.rst

@@ -88,6 +88,13 @@ is a strong connection between the error and its ``<input>``, as required by the
 ``form_label()`` internally. If you call to ``form_errors()`` in your template,
 you'll get the error messages displayed *twice*.
 
+.. tip::
+
+    Since form errors are rendered *inside* the ``<label>``, you cannot use CSS
+    ``:after`` to append an asterisk to the label, because it would be displayed
+    after the error message. Use the :ref:`label <reference-form-option-label-html>`
+    option instead.
+
 Checkboxes and Radios
 ---------------------
 

reference/forms/types/form.rst

@@ -170,6 +170,8 @@ of the form type tree (i.e. it cannot be used as a form type on its own).
 
 .. include:: /reference/forms/types/options/disabled.rst.inc
 
+.. _reference-form-option-label:
+
 .. include:: /reference/forms/types/options/label.rst.inc
 
 .. include:: /reference/forms/types/options/row_attr.rst.inc