minor #11456 Update Length constraint description (mb_strlen) (guilliamxavier)

This PR was merged into the 3.4 branch.

Discussion
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Update Length constraint description (mb_strlen)

The current implementation ([3.4] and [4.2]) uses only `mb_strlen` (after `mb_check_encoding`).

[3.4]: https://github.com/symfony/symfony/blob/3.4/src/Symfony/Component/Validator/Constraints/LengthValidator.php#L42-L44
[4.2]: https://github.com/symfony/symfony/blob/4.2/src/Symfony/Component/Validator/Constraints/LengthValidator.php#L43-L45

History: [years ago][1] the `grapheme_strlen` attempt was replaced with `iconv_strlen` (and `preg_replace`), [later on][2] the whole implementation (including plain `strlen`) was reduced to keep only `iconv_strlen`, and [finally][3] (but still years ago) `iconv_strlen` was replaced with `mb_check_encoding` + `mb_strlen`.

[1]: symfony/symfony@915fcd8
[2]: symfony/symfony@303f05b#diff-467932f4ce8a7100a8e0184cf9463e6a
[3]: symfony/symfony@27f5f81#diff-467932f4ce8a7100a8e0184cf9463e6a

Commits
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03bd47f Update Length constraint description (mb_strlen)

Author: javiereguiluz

reference/constraints/Length.rst

@@ -143,9 +143,8 @@ charset
 **type**: ``string``  **default**: ``UTF-8``
 
 The charset to be used when computing value's length. The
-:phpfunction:`grapheme_strlen` PHP function is used if available. If not,
-the :phpfunction:`mb_strlen` PHP function is used if available. If neither
-are available, the :phpfunction:`strlen` PHP function is used.
+:phpfunction:`mb_check_encoding` and :phpfunction:`mb_strlen`
+PHP functions are used.
 
 minMessage
 ~~~~~~~~~~