DOC: Fix default value for engine in pandas.eval

pandas/core/computation/eval.py

@@ -238,13 +238,14 @@ def eval(
         ``'python'`` parser to retain strict Python semantics.  See the
         :ref:`enhancing performance <enhancingperf.eval>` documentation for
         more details.
-    engine : {'python', 'numexpr'}, default 'numexpr'
+    engine : {'python', 'numexpr'}, optional, default None
 
         The engine used to evaluate the expression. Supported engines are
 
         - None : tries to use ``numexpr``, falls back to ``python``
-        - ``'numexpr'`` : This default engine evaluates pandas objects using
-          numexpr for large speed ups in complex expressions with large frames.
+        - ``'numexpr'`` : This is the default engine when ``numexpr`` is installed.
+          Evaluates pandas objects using numexpr for large speed ups in complex
+          expressions with large frames.
         - ``'python'`` : Performs operations as if you had ``eval``'d in top
           level python. This engine is generally not that useful.