minor #2473 Update number_format.rst (DarkBee)

This PR was submitted for the 2.x branch but it was merged into the 1.x branch instead (closes #2473).

Discussion
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Update number_format.rst

Added warning when trying to format negative numbers at literal

Commits
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40fdb9e3 Update number_format.rst

doc/filters/number_format.rst

@@ -17,6 +17,14 @@ separator using the additional arguments:
 .. code-block:: jinja
 
     {{ 9800.333|number_format(2, '.', ',') }}
+    
+If you want to format negative numbers, you will need to wrap the number with parantheses. This is due to the `precedence of operators`_ inside Twig
+
+.. code-block:: jinja
+
+    {{ -9800.333|number_format(2, '.', ',') }} {# outputs : -9 #}
+    {{ (-9800.333)|number_format(2, '.', ',') }} {# outputs : -9.800,33 #}
+
 
 If no formatting options are provided then Twig will use the default formatting
 options of:
@@ -46,3 +54,5 @@ Arguments
 * ``thousand_sep``:   The character(s) to use for the thousands separator
 
 .. _`number_format`: http://php.net/number_format
+.. _`precedence of operators`: https://twig.sensiolabs.org/doc/2.x/templates.html#expressions
+