minor #1422 Update slice.rst (1emming)

This PR was merged into the 1.16-dev branch.

Discussion
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Update slice.rst

Please state that Twig `slice` is  multi-byte safe since it uses `mb_substr` :)

Commits
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87165e7 Update slice.rst

doc/filters/slice.rst

@@ -41,7 +41,7 @@ As syntactic sugar, you can also use the ``[]`` notation:
     {{ '12345'[2:] }} {# will display "345" #}
 
 The ``slice`` filter works as the `array_slice`_ PHP function for arrays and
-`substr`_ for strings.
+`mb_substr`_ for strings with a fallback to `substr`_.
 
 If the start is non-negative, the sequence will start at that start in the
 variable. If start is negative, the sequence will start that far from the end
@@ -67,4 +67,5 @@ Arguments
 
 .. _`Traversable`: http://php.net/manual/en/class.traversable.php
 .. _`array_slice`: http://php.net/array_slice
+.. _`mb_substr` :  http://php.net/mb-substr
 .. _`substr`:      http://php.net/substr