minor #14776 [String] Feature 39178 (jontjs)

This PR was submitted for the 5.2 branch but it was merged into the 5.3-dev branch instead.

Discussion
----------

[String] Feature 39178

Docs for symfony/symfony#39178

Documentation update for [String] Feature 39178 "AsciiSlugger's symbolsMap should apply to all locales for a language".

Commits
-------

36a5c1a [String] Feature 39178

Author: javiereguiluz

components/string.rst

@@ -477,6 +477,12 @@ that only includes safe ASCII characters::
     $slug = $slugger->slug('10% or 5€');
     // $slug = '10-percent-or-5-euro'
 
+    // if there is no symbols map for your locale (e.g. 'en_GB') then the parent locale's symbols map
+    // will be used instead (i.e. 'en')
+    $slugger = new AsciiSlugger('en_GB', ['en' => ['%' => 'percent', '€' => 'euro']]);
+    $slug = $slugger->slug('10% or 5€');
+    // $slug = '10-percent-or-5-euro'
+
     // for more dynamic substitutions, pass a PHP closure instead of an array
     $slugger = new AsciiSlugger('en', function ($string, $locale) {
         return str_replace('❤️', 'love', $string);
@@ -490,6 +496,10 @@ that only includes safe ASCII characters::
 
     The feature to use a PHP closure to define substitutions was introduced in Symfony 5.2.
 
+.. versionadded:: 5.3
+
+    The feature to fallback to the parent locale's symbols map was introduced in Symfony 5.3.
+
 The separator between words is a dash (``-``) by default, but you can define
 another separator as the second argument::