added documentation for the trim filter

diff --git a/doc/filters/trim.rst b/doc/filters/trim.rst
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+``trim``
+========
+
+.. versionadded:: 1.6.2
+    The trim filter was added in Twig 1.6.2.
+
+The ``trim`` filter strips whitespace (or other characters) from the beginning
+and end of a string:
+
+.. code-block:: jinja
+
+    {{ '  I like Twig.  '|trim }}
+
+    {# outputs 'I like Twig.' #}
+
+    {{ '  I like Twig.'|trim('.') }}
+
+    {# outputs '  I like Twig' #}
+
+.. note::
+
+    Internally, Twig uses the PHP `trim`_ function.
+
+.. _`trim`: http://php.net/trim