minor #7990 Explain how to properly override choices (xabbuh)

This PR was merged into the 3.3 branch.

Discussion
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Explain how to properly override choices

Update the documentation for symfony/symfony#21919. This fixes #7779.

Commits
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a9f934e explain how to properly override choices

Author: xabbuh

reference/forms/types/country.rst

@@ -60,6 +60,12 @@ choices
 The country type defaults the ``choices`` option to the whole list of countries.
 The locale is used to translate the countries names.
 
+.. caution::
+
+    If you want to override the built-in choices of the country type, you
+    will also have to set the ``choice_loader`` option to ``null``. Not doing
+    so is deprecated since Symfony 3.3.
+
 Inherited Options
 -----------------
 

reference/forms/types/currency.rst

@@ -51,6 +51,12 @@ choices
 
 The choices option defaults to all currencies.
 
+.. caution::
+
+    If you want to override the built-in choices of the currency type, you
+    will also have to set the ``choice_loader`` option to ``null``. Not doing
+    so is deprecated since Symfony 3.3.
+
 Inherited Options
 -----------------
 

reference/forms/types/language.rst

@@ -61,6 +61,12 @@ choices
 The choices option defaults to all languages.
 The default locale is used to translate the languages names.
 
+.. caution::
+
+    If you want to override the built-in choices of the language type, you
+    will also have to set the ``choice_loader`` option to ``null``. Not doing
+    so is deprecated since Symfony 3.3.
+
 Inherited Options
 -----------------
 

reference/forms/types/locale.rst

@@ -63,6 +63,12 @@ choices
 The choices option defaults to all locales. It uses the default locale to
 specify the language.
 
+.. caution::
+
+    If you want to override the built-in choices of the locale type, you
+    will also have to set the ``choice_loader`` option to ``null``. Not doing
+    so is deprecated since Symfony 3.3.
+
 Inherited Options
 -----------------
 

reference/forms/types/timezone.rst

@@ -56,6 +56,12 @@ choices
 The Timezone type defaults the choices to all timezones returned by
 :phpmethod:`DateTimeZone::listIdentifiers`, broken down by continent.
 
+.. caution::
+
+    If you want to override the built-in choices of the timezone type, you
+    will also have to set the ``choice_loader`` option to ``null``. Not doing
+    so is deprecated since Symfony 3.3.
+
 Inherited Options
 -----------------