fixup! Store argument names in EvalFunction for documentation

Author: mlell

beanquery/query_compile.py

@@ -413,6 +413,7 @@ class EvalFunction(EvalNode):
         __outtype__: Output type of the function. None means no type
           annotation. To annotate that the function returns None, use
           types.NoneType.
+        __param_names__: List of argument names.
     """
     __slots__ = ('operands',)
 
@@ -422,6 +423,9 @@ class EvalFunction(EvalNode):
     # Output type annotation
     __outtype__ = None
 
+    # Input argument names for documentation.
+    __param_names__ = []
+
     def __init__(self, context, operands, dtype):
         super().__init__(dtype)
         self.context = context