[syahra712]

Fixes #573

The user guide says ne.validate() "returns None on success or raises an exception on invalid inputs", but it never raises: the exception object is returned, which matches the validate docstring and its Optional[Exception] return type. Someone following the guide with a try/except would silently treat invalid expressions as valid.

This rewords the sentence and adds a short example showing that the return value has to be checked. Docs only, no code changes.