[Form] Documenting new priority option

reference/forms/types/form.rst

@@ -48,6 +48,7 @@ on all types for which ``FormType`` is the parent.
 |                           | - `translation_domain`_                                            |
 |                           | - `label_translation_parameters`_                                  |
 |                           | - `attr_translation_parameters`_                                   |
+|                           | - `priority`_                                                      |
 +---------------------------+--------------------------------------------------------------------+
 | Default invalid message   | This value is not valid.                                           |
 +---------------------------+--------------------------------------------------------------------+
@@ -189,3 +190,5 @@ of the form type tree (i.e. it cannot be used as a form type on its own).
 .. include:: /reference/forms/types/options/label_translation_parameters.rst.inc
 
 .. include:: /reference/forms/types/options/attr_translation_parameters.rst.inc
+
+.. include:: /reference/forms/types/options/priority.rst.inc

reference/forms/types/options/priority.rst.inc

@@ -0,0 +1,10 @@
+priority
+~~~~~~~~
+
+**type**: ``integer`` **default**: ``0``
+
+    Allows you to set the field rendering priority. This option will affect
+    the view order only, sorting all fields in order of importance instead of
+    simply numerical order, i.e. fields with higher priority will be rendered
+    first, fields with lower priority will be rendered last and fields with
+    equal priority will keep the original order.