minor #11419 Documented the Inflector component (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #11419).

Discussion
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Documented the Inflector component

As reported in https://symfony.com/blog/new-in-symfony-4-3-better-inflector, this is a real component starting from Symfony 4.3, so we can document it.

Commits
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7c04fd1 Documented the Inflector component

Author: javiereguiluz

components/inflector.rst

@@ -0,0 +1,64 @@
+.. index::
+   single: Inflector
+   single: Components; Inflector
+
+The Inflector Component
+=======================
+
+    The Inflector component converts English words between their singular and
+    plural forms.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/inflector
+
+.. include:: /components/require_autoload.rst.inc
+
+When you May Need an Inflector
+------------------------------
+
+In some scenarios such as code generation and code introspection, it's usually
+required to convert words from/to singular/plural. For example, if you need to
+know which property is associated with an *adder* method, you must convert from
+plural to singular (``addStories()`` method -> ``$story`` property).
+
+Although most human languages define simple pluralization rules, they also
+define lots of exceptions. For example, the general rule in English is to add an
+``s`` at the end of the word (``book`` -> ``books``) but there are lots of
+exceptions even for common words (``woman`` -> ``women``, ``life`` -> ``lives``,
+``news`` -> ``news``, ``radius`` -> ``radii``, etc.)
+
+This component abstracts all those pluralization rules so you can convert
+from/to singular/plural with confidence. However, due to the complexity of the
+human languages, this component only provides support for the English language.
+
+Usage
+-----
+
+The Inflector component provides two static methods to convert from/to
+singular/plural::
+
+    use Symfony\Component\Inflector\Inflector;
+
+    Inflector::singularize('alumni');   // 'alumnus'
+    Inflector::singularize('knives');   // 'knife'
+    Inflector::singularize('mice');     // 'mouse'
+
+    Inflector::pluralize('grandchild'); // 'grandchildren'
+    Inflector::pluralize('news');       // 'news'
+    Inflector::pluralize('bacterium');  // 'bacteria'
+
+Sometimes it's not possible to determine a unique singular/plural form for the
+given word. In those cases, the methods return an array with all the possible
+forms::
+
+    use Symfony\Component\Inflector\Inflector;
+
+    Inflector::singularize('indices'); // ['index', 'indix', 'indice']
+    Inflector::singularize('leaves');  // ['leaf', 'leave', 'leaff']
+
+    Inflector::pluralize('matrix');    // ['matricies', 'matrixes']
+    Inflector::pluralize('person');    // ['persons', 'people']