Write docstrings for validate_fulfillment & validate_condition

```python
In [2]: cryptoconditions.validate_fulfillment?
Signature: cryptoconditions.validate_fulfillment(serialized_fulfillment, serialized_condition, message=None)
Docstring: <no docstring>

In [3]: cryptoconditions.validate_condition?
Signature: cryptoconditions.validate_condition(serialized_condition)
Docstring: <no docstring>
```