Add versionadded in file

Co-Authored-By: Oskar Stark <oskarstark@googlemail.com>

Author: MrYamous

components/expression_language/syntax.rst

@@ -21,6 +21,10 @@ The component supports:
 * **null** - ``null``
 * **exponential** - also known as scientific (e.g. ``1.99E+3`` or ``1e-2``)
 
+    .. versionadded:: 4.4
+    
+        The ``exponential`` literal was introduced in Symfony 4.4.
+
 .. caution::
 
     A backslash (``\``) must be escaped by 4 backslashes (``\\\\``) in a string