minor #12441 [Form] Added docs for alpha3 option in CountryType and LanguageType (creiner)

This PR was squashed before being merged into the 4.4 branch (closes #12441).

Discussion
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[Form] Added docs for alpha3 option in CountryType and LanguageType

Here are the documentation parts for the PR:
symfony/symfony#33791
Hope I didn't make any mistakes.. :-)

Commits
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377d71c [Form] Added docs for alpha3 option in CountryType and LanguageType

Author: javiereguiluz

reference/forms/types/country.rst

@@ -22,6 +22,7 @@ the option manually, but then you should just use the ``ChoiceType`` directly.
 | Rendered as | can be various tags (see :ref:`forms-reference-choice-tags`)          |
 +-------------+-----------------------------------------------------------------------+
 | Options     | - `choice_translation_locale`_                                        |
+|             | - `alpha3`_                                                           |
 +-------------+-----------------------------------------------------------------------+
 | Overridden  | - `choices`_                                                          |
 | options     |                                                                       |
@@ -64,6 +65,8 @@ Field Options
 
 .. include:: /reference/forms/types/options/choice_translation_locale.rst.inc
 
+.. include:: /reference/forms/types/options/alpha3.rst.inc
+
 Overridden Options
 ------------------
 

reference/forms/types/language.rst

@@ -24,6 +24,7 @@ manually, but then you should just use the ``ChoiceType`` directly.
 | Rendered as | can be various tags (see :ref:`forms-reference-choice-tags`)           |
 +-------------+------------------------------------------------------------------------+
 | Options     | - `choice_translation_locale`_                                         |
+|             | - `alpha3`_                                                            |
 +-------------+------------------------------------------------------------------------+
 | Overridden  | - `choices`_                                                           |
 | options     |                                                                        |
@@ -66,6 +67,8 @@ Field Options
 
 .. include:: /reference/forms/types/options/choice_translation_locale.rst.inc
 
+.. include:: /reference/forms/types/options/alpha3.rst.inc
+
 Overridden Options
 ------------------
 

reference/forms/types/options/alpha3.rst.inc

@@ -0,0 +1,7 @@
+alpha3
+~~~~~~
+
+**type**: ``boolean`` **default**: ``false``
+
+This option determines if the choice values should be in the alpha3 instead 
+of the alpha2 format.