Fix Function field docstring about context

src/marshmallow/fields.py

@@ -1900,13 +1900,13 @@ class Function(Field):
     :param serialize: A callable from which to retrieve the value.
         The function must take a single argument ``obj`` which is the object
         to be serialized. It can also optionally take a ``context`` argument,
-        which is a dictionary of context variables passed to the serializer.
+        which is the value of the current context returned by Context.get().
         If no callable is provided then the ```load_only``` flag will be set
         to True.
     :param deserialize: A callable from which to retrieve the value.
         The function must take a single argument ``value`` which is the value
         to be deserialized. It can also optionally take a ``context`` argument,
-        which is a dictionary of context variables passed to the deserializer.
+        which is the value of the current context returned by Context.get().
         If no callable is provided then ```value``` will be passed through
         unchanged.