minor #11449 Document the new Intl exists() methods (javiereguiluz)

This PR was merged into the master branch.

Discussion
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Document the new Intl exists() methods

As mentioned by our friend @ro0NL in this comment: https://symfony.com/blog/new-in-symfony-4-3-simpler-access-to-intl-data#comment-23227

Commits
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1ef964e Document the new Intl exists() methods

Author: javiereguiluz

components/intl.rst

@@ -240,6 +240,10 @@ which defaults to the current default locale::
     $languages = Languages::getNames('de');
     // => ['ab' => 'Abchasisch', ...]
 
+You can also check if a given language code is valid::
+
+    $isValidLanguage = Languages::exists($languageCode);
+
 .. versionadded:: 4.3
 
     The ``Languages`` class was introduced in Symfony 4.3.
@@ -259,6 +263,10 @@ for traditional Chinese)::
     $script = Scripts::getName('Hans');
     // => 'Simplified'
 
+You can also check if a given script code is valid::
+
+    $isValidScript = Scripts::exists($scriptCode);
+
 .. versionadded:: 4.3
 
     The ``Scrcipts`` class was introduced in Symfony 4.3.
@@ -288,6 +296,10 @@ which defaults to the current default locale::
     $countries = Regions::getNames('de');
     // => ['AF' => 'Afghanistan', ...]
 
+You can also check if a given region code is valid::
+
+    $isValidRegion = Regions::exists($regionCode);
+
 .. versionadded:: 4.3
 
     The ``Regions`` class was introduced in Symfony 4.3.
@@ -316,6 +328,10 @@ which defaults to the current default locale::
     $locales = Locales::getNames('de');
     // => ['af' => 'Afrikaans', ...]
 
+You can also check if a given locale code is valid::
+
+    $isValidLocale = Locales::exists($localeCode);
+
 .. versionadded:: 4.3
 
     The ``Locales`` class was introduced in Symfony 4.3.
@@ -352,6 +368,10 @@ the current default locale::
     $currencies = Currencies::getNames('de');
     // => ['AFN' => 'Afghanische Afghani', ...]
 
+You can also check if a given currency code is valid::
+
+    $isValidCurrency = Currencies::exists($currencyCode);
+
 .. versionadded:: 4.3
 
     The ``Currencies`` class was introduced in Symfony 4.3.