minor #2134 Expanded definition of empty (servel333)

This PR was submitted for the master branch but it was merged into the 1.x branch instead (closes #2134).

Discussion
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Expanded definition of empty

Based on the [source](https://github.com/twigphp/Twig/blob/f61bc838a97c26e2952af09070d1c4847eeedbe7/lib/Twig/Extension/Core.php#L1405), defined 'empty'. The comment in the code block is too subtle and I didn't notice it until I got to the edit page, and it also does not fully describe the definition of empty based on what the code does.

Commits
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9c6b8cb Expanded definition of empty

doc/tests/empty.rst

@@ -1,7 +1,7 @@
 ``empty``
 =========
 
-``empty`` checks if a variable is empty:
+``empty`` checks if a variable is an empty string, an empty array, an empty hash, exactly ``false``, exactly ``null``:
 
 .. code-block:: jinja
 
@@ -9,3 +9,6 @@
     {% if foo is empty %}
         ...
     {% endif %}
+
+.. _Countable: http://php.net/manual/en/class.countable.php
+.. _count: http://php.net/manual/en/function.count.php